Finalist: Staff of The New York Times

At critical turning points, Chinese authorities put secrecy and order ahead of openly confronting the growing crisis and risking public alarm or political embarrassment.

By Chris Buckley and Steven Lee Myers

WUHAN, China — A mysterious illness had stricken seven patients at a hospital, and a doctor tried to warn his medical school classmates. “Quarantined in the emergency department,” the doctor, Li Wenliang, wrote in an online chat group on Dec. 30, referring to patients.

“So frightening,” one recipient replied, before asking about the epidemic that began in China in 2002 and ultimately killed nearly 800 people. “Is SARS coming again?”

In the middle of the night, officials from the health authority in the central city of Wuhan summoned Dr. Li, demanding to know why he had shared the information. Three days later, the police compelled him to sign a statement that his warning constituted “illegal behavior.”

The illness was not SARS, but something similar: a coronavirus that is now on a relentless march outward from Wuhan, throughout the country and across the globe, killing at least 304 people in China and infecting more than 14,380 worldwide.

The government’s initial handling of the epidemic allowed the virus to gain a tenacious hold. At critical moments, officials chose to put secrecy and order ahead of openly confronting the growing crisis to avoid public alarm and political embarrassment.

A reconstruction of the crucial seven weeks between the appearance of the first symptoms in early December and the government’s decision to lock down the city, based on two dozen interviews with Wuhan residents, doctors and officials, on government statements and on Chinese media reports, points to decisions that delayed a concerted public health offensive.

In those weeks, the authorities silenced doctors and others for raising red flags. They played down the dangers to the public, leaving the city’s 11 million residents unaware they should protect themselves. They closed a food market where the virus was believed to have started, but didn’t broadly curb the wildlife trade.

Their reluctance to go public, in part, played to political motivations as local officials prepared for their annual congresses in January. Even as cases climbed, officials declared repeatedly that there had likely been no more infections.

By not moving aggressively to warn the public and medical professionals, public health experts say, the Chinese government lost one of its best chances to keep the disease from becoming an epidemic.

“This was an issue of inaction,” said Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations who studies China. “There was no action in Wuhan from the local health department to alert people to the threat.”

The first case, the details of which are limited and the specific date unknown, was in early December. By the time the authorities galvanized into action on Jan. 20, the disease had grown into a formidable threat.

It is now a global health emergency. It has triggered travel restrictions around the world, shaken financial markets and created perhaps the greatest challenge yet for China’s leader, Xi Jinping. The crisis could upend Mr. Xi’s agenda for months or longer, even undermining his vision of a political system that offers security and growth in return for submission to iron-fisted authoritarianism.

On the last day of 2019, after Dr. Li’s message was shared outside the group, the authorities focused on controlling the narrative. The police announced that they were investigating eight people for spreading rumors about the outbreak.

That same day, Wuhan’s health commission, its hand forced by those “rumors,” announced that 27 people were suffering from pneumonia of an unknown cause. Its statement said there was no need to be alarmed.

“The disease is preventable and controllable,” the statement said.

Dr. Li, an ophthalmologist, went back to work after being reprimanded. On Jan. 10, he treated a woman for glaucoma. He did not know she had already been infected with the coronavirus, probably by her daughter. They both became sick. So would he.

Hazmat Suits and Disinfectants

Hu Xiaohu, who sold processed pork in the Huanan Seafood Wholesale Market, sensed by late December that something was amiss. Workers were coming down with nagging fevers. No one knew why but, Mr. Hu said, several were in hospital quarantine.

The market occupies much of a block in a newer part of the city, sitting incongruously near apartment buildings and shops catering to the growing middle class. It is a warren of stalls selling meats, poultry and fish, as well as more exotic fare, including live reptiles and wild game that some in China prize as delicacies. According to a report by the city’s center for disease control, sanitation was dismal, with poor ventilation and garbage piled on wet floors.

In hospitals, doctors and nurses were puzzled to see a cluster of patients with symptoms of a viral pneumonia that did not respond to the usual treatments. They soon noticed that many patients had one thing in common: They worked in Huanan market.

On Jan. 1, police officers showed up at the market, along with public health officials, and shut it down. Local officials issued a notice that the market was undergoing an environmental and hygienic cleanup related to the pneumonia outbreak. That morning, workers in hazmat suits moved in, washing out stalls and spraying disinfectants.

It was, for the public, the first visible government response to contain the disease. The day before, on Dec. 31, national authorities had alerted the World Health Organization’s office in Beijing of an outbreak.

City officials struck optimistic notes in their announcements. They suggested they had stopped the virus at its source. The cluster of illnesses was limited. There was no evidence the virus spread between humans.

“Projecting optimism and confidence, if you don’t have the data, is a very dangerous strategy,” said Alexandra Phelan, a faculty research instructor in the department of microbiology and immunology at Georgetown University.

“It undermines the legitimacy of the government in messaging,” she added. “And public health is dependent on public trust.”

Nine days after the market closed, a man who shopped there regularly became the first fatality of the disease, according to a report by the Wuhan Health Commission, the agency that oversees public health and sanitation. The 61-year-old, identified by his last name, Zeng, already had chronic liver disease and a tumor in his abdomen, and had checked into Wuhan Puren Hospital with a raging fever and difficulty breathing.

The authorities disclosed the man’s death two days after it happened. They did not mention a crucial detail in understanding the course of the epidemic. Mr. Zeng’s wife had developed symptoms five days after he did.

She had never visited the market.

The Race to Identify a Killer

About 20 miles from the market, scientists at the Wuhan Institute of Virology were studying samples from the patients checking into the city’s hospitals. One of the scientists, Zheng-Li Shi, was part of the team that tracked down the origins of the SARS virus, which emerged in the southern province of Guangdong in 2002.

As the public remained largely in the dark about the virus, she and her colleagues quickly pieced together that the new outbreak was related to SARS. The genetic composition suggested a common initial host: bats. The SARS epidemic began when a coronavirus jumped from bats to Asian palm civets, a catlike creature that is legally raised and consumed. It was likely that this new coronavirus had followed a similar path — possibly somewhere in or on the way to the Huanan market or another market like it.

Around the same time, Dr. Li and other medical professionals in Wuhan started trying to provide warnings to colleagues and others when the government did not. Lu Xiaohong, the head of gastroenterology at City Hospital No. 5, told China Youth Daily that she had heard by Dec. 25 that the disease was spreading among medical workers — a full three weeks before the authorities would acknowledge the fact. She did not go public with her concerns, but privately warned a school near another market.

By the first week of January, the emergency ward in Hospital No. 5 was filling; the cases included members of the same family, making it clear that the disease was spreading through human contact, which the government had said was not likely.

No one realized, the doctor said, that it was as serious as it would become until it was too late to stop it.

“I realized that we had underestimated the enemy,” she said.

At the Institute of Virology, Dr. Shi and her colleagues isolated the genetic sequence and the viral strain during the first week of January. They used samples from seven of the first patients, six of them vendors at the market.

On Jan. 7, the institute’s scientists gave the new coronavirus its identity and began referring to it by the technical shorthand 2019-nCoV. Four days later, the team shared the virus’s genetic makeup in a public database for scientists everywhere to use.

That allowed scientists around the world to study the virus and swiftly share their findings. As the scientific community moved quickly to devise a test for exposure, political leaders remained reluctant to act.

‘Politics Is Always No. 1’

As the virus spread in early January, the mayor of Wuhan, Zhou Xianwang, was touting futuristic health care plans for the city.

It was China’s political season, when officials gather for annual meetings of People’s Congresses — the Communist Party-run legislatures that discuss and praise policies. It is not a time for bad news.

When Mr. Zhou delivered his annual report to the city’s People’s Congress on Jan. 7 against a backdrop of bright red national flags, he promised the city top-class medical schools, a World Health Expo, and a futuristic industry park for medical companies. Not once did he or any other city or provincial leader publicly mention the viral outbreak.

“Stressing politics is always No. 1,” the governor of Hubei, Wang Xiaodong, told officials on Jan. 17, citing Mr. Xi’s precepts of top-down obedience. “Political issues are at any time the most fundamental major issues.”

Shortly after, Wuhan went ahead with a massive annual potluck banquet for 40,000 families from a city precinct, which critics later cited as evidence that local leaders took the virus far too lightly.

As the congress was taking place, the health commission’s daily updates on the outbreak said again and again that there were no new cases of infection, no firm evidence of human transmission and no infection of medical workers.

“We knew this was not the case!” said a complaint later filed with the National Health Commission on a government website. The anonymous author said he was a doctor in Wuhan and described a surge in unusual chest illnesses beginning Jan. 12.

Officials told doctors at a top city hospital “don’t use the words viral pneumonia on the image reports,” according to the complaint, which has since been removed. People were complacent, “thinking that if the official reports had nothing, then we were exaggerating,” the doctor explained.

Even those stricken felt lulled into complacency.

When Dong Guanghe developed a fever on Jan. 8 in Wuhan, his family was not alarmed, his daughter said. He was treated in the hospital and sent home. Then, 10 days later, Mr. Dong’s wife fell ill with similar symptoms.

“The news said nothing about the severity of the epidemic,” said the daughter, Dong Mingjing. “I thought that my dad had a common cold.”

The government’s efforts to minimize public disclosure persuaded more than just untrained citizens.

“If there are no new cases in the next few days, the outbreak is over,” Guan Yi, a respected professor of infectious diseases at the University of Hong Kong, said on Jan. 15.

The World Health Organization’s statements during this period echoed the reassuring words of Chinese officials.

It had spread. Thailand reported the first confirmed case outside China on Jan. 13.

A City Besieged

The first deaths and the spread of the disease abroad appeared to grab the attention of the top authorities in Beijing. The national government dispatched Zhong Nanshan, a renowned and now-semiretired epidemiologist who was instrumental in the fight against SARS, to Wuhan to assess the situation.

He arrived on Jan. 18, just as the tone of local officials was shifting markedly. A health conference in Hubei Province that day called on medical workers to make the disease a priority. An internal document from Wuhan Union Hospital warned its employees that the coronavirus could be spread through saliva.

On Jan. 20, more than a month after the first symptoms spread, the current of anxiety that had been steadily gaining strength exploded into public. Dr. Zhong announced in an interview on state television that there was no doubt that the coronavirus spread with human contact. Worse, one patient had infected at least 14 medical personnel.

Mr. Xi, fresh from a state visit to Myanmar, made his first public statement about the outbreak, issuing a brief set of instructions.

It was only with the order from Mr. Xi that the bureaucracy leapt into action. At that point the death toll was three; in the next 11 days, it would rise above 200.

In Wuhan, the city banned tour groups from visiting. Residents began pulling on masks.

Guan Yi, the Hong Kong expert who had earlier voiced optimism that the outbreak could level off, was now alarmed. He dropped by one of the city’s other food markets and was shocked by the complacency, he said. He told city officials that the epidemic was “already beyond control” and would leave. “I hurriedly booked a departure,” Dr. Guan told Caixin, a Chinese news organization.

Two days later, the city announced that it was shutting itself down, a move that could only have been approved by Beijing.

In Wuhan, many residents said they did not grasp the gravity of the epidemic until the lockdown. The mass alarm that officials feared at the start became a reality, heightened by the previous paucity of information.

Crowds of people crushed the airport and train stations to get out before the deadline fell on the morning of Jan. 23. Hospitals were packed with people desperate to know if they, too, were infected.

“We didn’t wear masks at work. That would have frightened off customers,” Yu Haiyan, a waitress from rural Hubei, said of the days before the shutdown. “When they closed off Wuhan, only then did I think, ‘Oh, this is really serious, this is not some average virus.’”

Wuhan’s mayor, Zhou Xianwang, later took responsibility for the delay in reporting the scale of the epidemic, but said he was hampered by the national law on infectious diseases. That law allows provincial governments to declare an epidemic only after receiving central government approval. “After I receive information, I can only release it when I’m authorized,” he said.

The official reflex for suppressing discomforting information now appears to be cracking, as officials at various levels seek to shift blame for the government’s response.

With the crisis worsening, Dr. Li’s efforts are no longer viewed as reckless. A commentary on the social media account of the Supreme People’s Court criticized the police for investigating people for circulating rumors.

“It might have been a better way to prevent and control the new coronavirus today if the public had believed the ‘rumor’ then and started to wear masks and carry out sanitary measures and avoid the wild animal market,” the commentary said.

Dr. Li is 34 and has a child. He and his wife are expecting a second in the summer. He is now recovering from the virus in the hospital where he worked. In an interview via text messages, he said he felt aggrieved by the police actions.

“If the officials had disclosed information about the epidemic earlier,” he said, “I think it would have been a lot better. There should be more openness and transparency.”

This article is based on reporting and research by Elsie Chen, Sheri Fink, Claire Fu, Javier Hernandez, Zoe Mou, Amy Qin, Knvul Sheikh, Amber Wang, Yiwei Wang, Sui-Lee Wee, Li Yuan, Albee Zhang and Raymond Zhong.

Symptomless transmission makes the coronavirus far harder to fight. But health officials dismissed the risk for months, pushing misleading and contradictory claims in the face of mounting evidence.

By Matt Apuzzo, Selam Gebrekidan and David D. Kirkpatrick

MUNICH — Dr. Camilla Rothe was about to leave for dinner when the government laboratory called with the surprising test result. Positive. It was Jan. 27. She had just discovered Germany’s first case of the new coronavirus.

But the diagnosis made no sense. Her patient, a businessman from a nearby auto parts company, could have been infected by only one person: a colleague visiting from China. And that colleague should not have been contagious.

The visitor had seemed perfectly healthy during her stay in Germany. No coughing or sneezing, no signs of fatigue or fever during two days of long meetings. She told colleagues that she had started feeling ill after the flight back to China. Days later, she tested positive for the coronavirus.

Scientists at the time believed that only people with symptoms could spread the coronavirus. They assumed it acted like its genetic cousin, SARS.

“People who know much more about coronaviruses than I do were absolutely sure,” recalled Dr. Rothe, an infectious disease specialist at Munich University Hospital.

But if the experts were wrong, if the virus could spread from seemingly healthy carriers or people who had not yet developed symptoms, the ramifications were potentially catastrophic. Public-awareness campaigns, airport screening and stay-home-if-you’re sick policies might not stop it. More aggressive measures might be required — ordering healthy people to wear masks, for instance, or restricting international travel.

Dr. Rothe and her colleagues were among the first to warn the world. But even as evidence accumulated from other scientists, leading health officials expressed unwavering confidence that symptomless spreading was not important.

In the days and weeks to come, politicians, public health officials and rival academics disparaged or ignored the Munich team. Some actively worked to undermine the warnings at a crucial moment, as the disease was spreading unnoticed in French churches, Italian soccer stadiums and Austrian ski bars. A cruise ship, the Diamond Princess, would become a deadly harbinger of symptomless spreading.

Interviews with doctors and public health officials in more than a dozen countries show that for two crucial months — and in the face of mounting genetic evidence — Western health officials and political leaders played down or denied the risk of symptomless spreading. Leading health agencies including the World Health Organization and the European Center for Disease Prevention and Control provided contradictory and sometimes misleading advice. A crucial public health discussion devolved into a semantic debate over what to call infected people without clear symptoms.

The two-month delay was a product of faulty scientific assumptions, academic rivalries and, perhaps most important, a reluctance to accept that containing the virus would take drastic measures. The resistance to emerging evidence was one part of the world’s sluggish response to the virus.

It is impossible to calculate the human toll of that delay, but models suggest that earlier, aggressive action might have saved tens of thousands of lives. Countries like Singapore and Australia, which used testing and contact-tracing and moved swiftly to quarantine seemingly healthy travelers, fared far better than those that did not.

It is now widely accepted that seemingly healthy people can spread the virus, though uncertainty remains over how much they have contributed to the pandemic. Though estimates vary, models using data from Hong Kong, Singapore and China suggest that 30 to 60 percent of spreading occurs when people have no symptoms.

“This was, I think, a very simple truth,” Dr. Rothe said. “I was surprised that it would cause such a storm. I can’t explain it.”

Even now, with more than 9 million cases around the world, and a death toll approaching 500,000, Covid-19 remains an unsolved riddle. It is too soon to know whether the worst has passed, or if a second global wave of infections is about to crash down. But it is clear that an array of countries, from secretive regimes to overconfident democracies, have fumbled their response, misjudged the virus and ignored their own emergency plans.

It is also painfully clear that time was a critical commodity in curbing the virus — and that too much of it was wasted.

‘She Was Not Ill’

On the night of Germany’s first positive test, the virus had seemed far away. Fewer than 100 fatalities had been reported worldwide. Italy, which would become Europe’s ground zero, would not record its first cases for another three days.

A few reports out of China had already suggested the possibility of symptomless spreading. But nobody had proved it could happen.

That night, Dr. Rothe tapped out an email to a few dozen doctors and public health officials.

“Infections can actually be transmitted during the incubation period,” she wrote.

Three more employees from the auto parts company, Webasto, tested positive the following day. Their symptoms were so mild that, normally, it’s likely that none would have been flagged for testing, or have thought to stay at home.

Dr. Rothe decided she had to sound the alarm. Her boss, Dr. Michael Hoelscher, dashed off an email to The New England Journal of Medicine. “We believe that this observation is of utmost importance,” he wrote.

Editors responded immediately. How soon could they see the paper?

The next morning, Jan. 30, public health officials interviewed the Chinese businesswoman by phone. Hospitalized in Shanghai, she explained that she’d started feeling sick on the flight home. Looking back, maybe she’d had some mild aches or fatigue, but she had chalked them up to a long day of travel.

“From her perspective, she was not ill,” said Nadine Schian, a Webasto spokeswoman who was on the call. “She said, ‘OK, I felt tired. But I’ve been in Germany a lot of times before and I always have jet lag.’”

When the health officials described the call, Dr. Rothe and Dr. Hoelscher quickly finished and submitted their article. Dr. Rothe did not talk to the patient herself but said she relied on the health authority summary.

Within hours, it was online. It was a modest clinical observation at a key time. Just days earlier, the World Health Organization had said it needed more information about this very topic.

What the authors did not know, however, was that in a suburb 20 minutes away, another group of doctors had also been rushing to publish a report. Neither knew what the other was working on, a seemingly small academic rift that would have global implications.

Academic Hairsplitting

The second group was made up of officials with the Bavarian health authority and Germany’s national health agency, known as the Robert Koch Institute. Inside a suburban office, doctors unfurled mural paper and traced infection routes using colored pens.

Their team, led by the Bavarian epidemiologist Dr. Merle Böhmer, submitted an article to The Lancet, another premier medical journal. But the Munich hospital group had scooped them by three hours. Dr. Böhmer said her team’s article, which went unpublished as a result, had reached similar conclusions but worded them slightly differently.

Dr. Rothe had written that patients appeared to be contagious before the onset of any symptoms. The government team had written that patients appeared to be contagious before the onset of full symptoms — at a time when symptoms were so mild that people might not even recognize them.

The Chinese woman, for example, had woken up in the middle of the night feeling jet-lagged. Wanting to be sharp for her meetings, she took a Chinese medicine called 999 — containing the equivalent of a Tylenol tablet — and went back to bed.

Perhaps that had masked a mild fever? Perhaps her jet lag was actually fatigue? She had reached for a shawl during a meeting. Maybe that was a sign of chills?

After two lengthy phone calls with the woman, doctors at the Robert Koch Institute were convinced that she had simply failed to recognize her symptoms. They wrote to the editor of The New England Journal of Medicine, casting doubt on Dr. Rothe’s findings.

Editors there decided that the dispute amounted to hairsplitting. If it took a lengthy interview to identify symptoms, how could anyone be expected to do it in the real world?

“The question was whether she had something consistent with Covid-19 or that anyone would have recognized at the time was Covid-19,” said Dr. Eric Rubin, the journal’s editor.

“The answer seemed to be no.”

The journal did not publish the letter. But that would not be the end of it.

That weekend, Andreas Zapf, the head of the Bavarian health authority, called Dr. Hoelscher of the Munich clinic. “Look, the people in Berlin are very angry about your publication,” Dr. Zapf said, according to Dr. Hoelscher.

He suggested changing the wording of Dr. Rothe’s report and replacing her name with those of members of the government task force, Dr. Hoelscher said. He refused.

The health agency would not discuss the phone call.

Until then, Dr. Hoelscher said, their report had seemed straightforward. Now it was clear: “Politically, this was a major, major issue.”

‘A Complete Tsunami’

On Monday, Feb. 3, the journal Science published an article calling Dr. Rothe’s report “flawed.” Science reported that the Robert Koch Institute had written to the New England Journal to dispute her findings and correct an error.

The Robert Koch Institute declined repeated interview requests over several weeks and did not answer written questions.

Dr. Rothe’s report quickly became a symbol of rushed research. Scientists said she should have talked to the Chinese patient herself before publishing, and that the omission had undermined her team’s work. On Twitter, she and her colleagues were disparaged by scientists and armchair experts alike.

“It broke over us like a complete tsunami,” Dr. Hoelscher said.

The controversy also overshadowed another crucial development out of Munich.

The next morning, Dr. Clemens-Martin Wendtner made a startling announcement. Dr. Wendtner was overseeing treatment of Munich’s Covid-19 patients — there were eight now — and had taken swabs from each.

He discovered the virus in the nose and throat at much higher levels, and far earlier, than had been observed in SARS patients. That meant it probably could spread before people knew they were sick.

But the Science story drowned that news out. If Dr. Rothe’s paper had implied that governments might need to do more against Covid-19, the pushback from the Robert Koch Institute was an implicit defense of the conventional thinking.

Sweden’s public health agency declared that Dr. Rothe’s report had contained major errors. The agency’s website said, unequivocally, that “there is no evidence that people are infectious during the incubation period” — an assertion that would remain online in some form for months.

French health officials, too, left no room for debate: “A person is contagious only when symptoms appear,” a government flyer read. “No symptoms = no risk of being contagious.”

As Dr. Rothe and Dr. Hoelscher reeled from the criticism, Japanese doctors were preparing to board the Diamond Princess cruise ship. A former passenger had tested positive for coronavirus.

Yet on the ship, parties continued. The infected passenger had been off the ship for days, after all. And he hadn’t reported symptoms while onboard.

A Semantic Debate

Immediately after Dr. Rothe’s report, the World Health Organization had noted that patients might transmit the virus before showing symptoms. But the organization also underscored a point that it continues to make: Patients with symptoms are the main drivers of the epidemic.

Once the Science article was published, however, the organization waded directly into the debate on Dr. Rothe’s work. On Tuesday, Feb. 4, Dr. Sylvie Briand, the agency’s chief of infectious disease preparedness, tweeted a link to the Science article, calling Dr. Rothe’s report flawed.

With that tweet, the W.H.O. focused on a semantic distinction that would cloud discussion for months: Was the patient asymptomatic, meaning she would never show symptoms? Or pre-symptomatic, meaning she became sick later? Or, even more confusing, oligo-symptomatic, meaning that she had symptoms so mild that she didn’t recognize them?

To some doctors, the focus on these arcane distinctions felt like whistling in the graveyard. A person who feels healthy has no way to know that she is carrying a virus or is about to become sick. Airport temperature checks will not catch these people. Neither will asking them about their symptoms or telling them to stay home when they feel ill.

The W.H.O. later said that the tweet had not been intended as a criticism.

One group paid little attention to this brewing debate: the Munich-area doctors working to contain the cluster at the auto parts company. They spoke daily with potentially sick people, monitoring their symptoms and tracking their contacts.

“For us, it was pretty soon clear that this disease can be transmitted before symptoms,” said Dr. Monika Wirth, who tracked contacts in the nearby county of Fürstenfeldbruck.

Dr. Rothe, though, was shaken. She could not understand why much of the scientific establishment seemed eager to play down the risk.

“All you need is a pair of eyes,” she said. “You don’t need rocket-science virology.”

But she remained confident.

“We will be proven right,” she told Dr. Hoelscher.

That night, Dr. Rothe received an email from Dr. Michael Libman, an infectious-disease specialist in Montreal. He thought that criticism of the paper amounted to semantics. Her paper had convinced him of something: “The disease will most likely eventually spread around the world.”

Political Paralysis

On Feb. 4, Britain’s emergency scientific committee met and, while its experts did not rule out the possibility of symptomless transmission, nobody put much stock in Dr. Rothe’s paper.

“It was very much a hearsay study,” said Wendy Barclay, a virologist and member of the committee, known as the Scientific Advisory Group for Emergencies. “In the absence of real robust epidemiology and tracing, it isn’t obvious until you see the data.”

The data would soon arrive, and from an unexpected source. Dr. Böhmer, from the Bavarian health team, received a startling phone call in the second week of February.

Virologists had discovered a subtle genetic mutation in the infections of two patients from the Munich cluster. They had crossed paths for the briefest of moments, one passing a saltshaker to the other in the company cafeteria, when neither had symptoms. Their shared mutation made it clear that one had infected the other.

Dr. Böhmer had been skeptical of symptomless spreading. But now, there was no doubt: “It can only be explained with pre-symptomatic transmission,” Dr. Böhmer said.

Now it was Dr. Böhmer who sounded the alarm. She said she promptly shared the finding, and its significance, with the W.H.O. and the European Center for Disease Prevention and Control.

Neither organization included the discovery in its regular reports.

A week after receiving Dr. Böhmer’s information, European health officials were still declaring: “We are still unsure whether mild or asymptomatic cases can transmit the virus.” There was no mention of the genetic evidence.

W.H.O. officials say the genetic discovery informed their thinking, but they made no announcement of it. European health officials say the German information was one early piece of an emerging picture that they were still piecing together.

The doctors in Munich were increasingly frustrated and confused by the World Health Organization. First, the group wrongly credited the Chinese government with alerting the German authorities to the first infection. Government officials and doctors say the auto parts company itself sounded the alarm.

Then, the World Health Organization’s emergency director, Dr. Michael Ryan, said on Feb. 27 that the significance of symptomless spreading was becoming a myth. And Dr. Maria Van Kerkhove, the organization’s technical lead on coronavirus response, suggested it was nothing to worry about.

“It’s rare but possible,” she said. “It’s very rare.”

The agency still maintains that people who cough or sneeze are more contagious than people who don’t. But there is no scientific consensus on how significant this difference is or how it affects the spread of virus.

And so, with evidence mounting, the Munich team could not understand how the W.H.O. could be so sure that symptomless spreading was insignificant.

“At this point, for us it was clear,” said Dr. Wendtner, the senior doctor overseeing treatment of the Covid-19 patients. “This was a misleading statement by the W.HO.”

‘If This Is True, We’re in Trouble’

The Munich cluster was not the only warning.

The Chinese health authorities had explicitly cautioned that patients were contagious before showing symptoms. A Japanese bus driver was infected while transporting seemingly healthy tourists from Wuhan.

And by the middle of February, 355 people aboard the Diamond Princess cruise ship had tested positive. About a third of the infected passengers and staff had no symptoms.

But public health officials saw danger in promoting the risk of silent spreaders. If quarantining sick people and tracing their contacts could not reliably contain the disease, governments might abandon those efforts altogether.

In Sweden and Britain, for example, discussion swirled about enduring the epidemic until the population obtained “herd immunity.” Public health officials worried that might lead to overwhelmed hospitals and needless deaths.

Plus, preventing silent spreading required aggressive, widespread testing that was then impossible for most countries.

“It’s not like we had some easy alternative,” said Dr. Libman, the Canadian doctor. “The message was basically: ‘If this is true, we’re in trouble.’”

European health officials say they were reluctant to acknowledge silent spreading because the evidence was trickling in and the consequences of a false alarm would have been severe. “These reports are seen everywhere, all over the world,” said Dr. Josep Jansa, a senior European Union health official. “Whatever we put out, there’s no way back.”

Looking back, health officials should have said that, yes, symptomless spreading was happening and they did not understand how prevalent it was, said Dr. Agoritsa Baka, a senior European Union doctor.

But doing that, she said, would have amounted to an implicit warning to countries: What you’re doing might not be enough.

‘Stop Buying Masks!’

While public health officials hesitated, some doctors acted. At a conference in Seattle in mid-February, Jeffrey Shaman, a Columbia University professor, said his research suggested that Covid-19’s rapid spread could only be explained if there were infectious patients with unremarkable symptoms or no symptoms at all.

In the audience that day was Steven Chu, the Nobel-winning physicist and former U.S. energy secretary. “If left to its own devices, this disease will spread through the whole population,” he remembers Professor Shaman warning.

Afterward, Dr. Chu began insisting that healthy colleagues at his Stanford University laboratory wear masks. Doctors in Cambridge, England, concluded that asymptomatic transmission was a big source of infection and advised local health workers and patients to wear masks, well before the British government acknowledged the risk of silent spreaders.

The American authorities, faced with a shortage, actively discouraged the public from buying masks. “Seriously people — STOP BUYING MASKS!” Surgeon General Jerome M. Adams tweeted on Feb. 29.

By early March, while the World Health Organization continued pressing the case that symptom-free transmission was rare, science was breaking in the other direction.

Researchers in Hong Kong estimated that 44 percent of Covid-19 transmission occurred before symptoms began, an estimate that was in line with a British study that put that number as high as 50 percent.

The Hong Kong study concluded that people became infectious about two days before their illness emerged, with a peak on their first day of symptoms. By the time patients felt the first headache or scratch in the throat, they might have been spreading the disease for days.

In Belgium, doctors saw that math in action, as Covid-19 tore through nursing homes, killing nearly 5,000 people.

“We thought that by monitoring symptoms and asking sick people to stay at home, we would be able to manage the spread,” said Steven Van Gucht, the head of Belgium’s Covid-19 scientific committee. “It came in through people with hardly any symptoms.”

More than 700 people aboard the Diamond Princess were sickened. Fourteen died. Researchers estimate that most of the infection occurred early on, while seemingly healthy passengers socialized and partied.

Government scientists in Britain concluded in late April that 5 to 6 percent of symptomless health care workers were infected and might have been spreading the virus.

In Munich, Dr. Hoelscher has asked himself many times whether things would have been different if world leaders had taken the issue seriously earlier. He compared their response to a rabbit stumbling upon a poisonous snake.

“We were watching that snake and were somehow paralyzed,” he said.

Acceptance. Or Not.

As the research coalesced in March, European health officials were convinced.

“OK, this is really a big issue,” Dr. Baka recalled thinking. “It plays a big role in the transmission.”

By the end of the month, the U.S. Centers for Disease Control announced it was rethinking its policy on masks. It concluded that up to 25 percent of patients might have no symptoms.

Since then, the C.D.C., governments around the world and, finally, the World Health Organization have recommended that people wear masks in public.

Still, the W.H.O. is sending confusing signals. Earlier this month, Dr. Van Kerkhove, the technical lead, repeated that transmission from asymptomatic patients was “very rare.” After an outcry from doctors, the agency said there had been a misunderstanding.

“In all honesty, we don’t have a clear picture on this yet,” Dr. Van Kerkhove said. She said she had been referring to a few studies showing limited transmission from asymptomatic patients.

Recent internet ads confused the matter even more. A Google search in mid-June for studies on asymptomatic transmission returned a W.H.O. advertisement titled: “People With No Symptoms — Rarely Spread Coronavirus.”

Clicking on the link, however, offered a much more nuanced picture: “Some reports have indicated that people with no symptoms can transmit the virus. It is not yet known how often it happens.”

After The Times asked about those discrepancies, the organization removed the advertisements.

Back in Munich, there is little doubt left. Dr. Böhmer, the Bavarian government doctor, published a study in The Lancet last month that relied on extensive interviews and genetic information to methodically track every case in the cluster.

In the months after Dr. Rothe swabbed her first patient, 16 infected people were identified and caught early. All survived. Aggressive testing and flawless contact-tracing contained the spread.

Dr. Böhmer’s study found “substantial” transmission from people with no symptoms or exceptionally mild, nonspecific symptoms.

Dr. Rothe and her colleagues got a footnote.

The young medical professionals, who worked long hours on the front lines in Wuhan, first came down with fevers. Within weeks, both were in hospital beds, hooked up to IVs or oxygen machines.

By Sui-Lee Wee and Vivian Wang

The young mothers didn’t tell their children they had the coronavirus. Mama was working hard, they said, to save sick people.

Instead, Deng Danjing and Xia Sisi were fighting for their lives in the same hospitals where they worked, weak from fever and gasping for breath. Within a matter of weeks, they had gone from healthy medical professionals on the front lines of the epidemic in Wuhan, China, to coronavirus patients in critical condition.

The world is still struggling to fully understand the new virus, its symptoms, spread and sources. For some, it can feel like a common cold. For others, it is a deadly infection that ravages the lungs and pushes the immune system into overdrive, destroying even healthy cells. The difference between life and death can depend on the patient’s health, age and access to care — although not always.

The virus has infected more than 132,000 globally. A vast majority of cases have been mild, with limited symptoms. But the virus’s progression can be quick, at which point the chances of survival plummet. At one recent count, around 68,000 people had recovered, while nearly 5,000 had died.

The fates of Ms. Deng and Dr. Xia reflect the unpredictable nature of a virus that affects everyone differently, at times defying statistical averages and scientific research.

As the new year opened in China, the women were leading remarkably similar lives. Both were 29 years old. Both were married, each with a young child on whom she doted.

Ms. Deng, a nurse, had worked for three years at Wuhan No. 7 Hospital, in the city where she grew up and where the coronavirus pandemic began. Her mother was a nurse there, too, and in their free time they watched movies or shopped together. Ms. Deng’s favorite activity was playing with her two pet kittens, Fat Tiger and Little White, the second of which she had rescued just three months before falling sick.

Dr. Xia, a gastroenterologist, also came from a family of medical professionals. As a young child, she had accompanied her mother, a nurse, to work. She joined the Union Jiangbei Hospital of Wuhan in 2015 and was the youngest doctor in her department. Her colleagues called her Little Sisi, or Little Sweetie, because she always had a smile for them. She loved Sichuan hot pot, a dish famous for its numbingly spicy broth.

When a mysterious new virus struck the city, the women began working long hours, treating a seemingly endless flood of patients. They took precautions to protect themselves. But they succumbed to the infection, the highly contagious virus burrowing deep into their lungs, causing fever and pneumonia. In the hospital, each took a turn for the worse.

One recovered. One did not.

Symptoms: Onset of Virus and Hospitalization

The symptoms came on suddenly.

Dr. Xia had ended her night shift on Jan. 14 when she was called back to attend to a patient — a 76-year-old man with suspected coronavirus. She dropped in frequently to check in on him.

Five days later, she started feeling unwell. Exhausted, she took a two-hour nap at home, then checked her temperature: It was 102 degrees. Her chest felt tight.

A few weeks later, in early February, Ms. Deng, the nurse, was preparing to eat dinner at the hospital office, when the sight of food left her nauseated. She brushed the feeling aside, figuring she was worn out by work. She had spent the beginning of the outbreak visiting the families of confirmed patients and teaching them to disinfect their homes.

After forcing down some food, Ms. Deng went home to shower and then, feeling groggy, took a nap. When she woke up, her temperature was 100 degrees.

Fever is the most common symptom of the coronavirus, seen in nearly 90 percent of patients. About a fifth of people experience shortness of breath, often including a cough and congestion. Many also feel fatigued.

Both women rushed to see doctors. Chest scans showed damage to their lungs, a telltale sign of the coronavirus that is present in at least 85 percent of patients, according to one study.

In particular, Ms. Deng’s CT scan showed what the doctor called ground-glass opacities on her lower right lung — hazy spots that indicated fluid or inflammation around her airways.

The hospital had no space, so Ms. Deng checked into a hotel to avoid infecting her husband and 5-year-old daughter. She sweated through the night. At one point, her calf twitched. In the morning, she was admitted to the hospital. Her throat was swabbed for a genetic test, which confirmed she had the coronavirus.

Her room in a newly opened staff ward was small, with two cots and a number assigned to each one. Ms. Deng was in Bed 28. Her roommate was a colleague who also had the virus.

At Jiangbei Hospital, 18 miles away, Dr. Xia was struggling to breathe. She was placed in an isolation ward, treated by doctors and nurses who wore protective suits and safety goggles. The room was cold.

Treatment: Day 1, Hospitalization Begins

When Ms. Deng checked into the hospital, she tried to stay upbeat. She texted her husband, urging him to wear a mask, even at home, and to clean all their bowls and chopsticks with boiling water or throw them out.

Her husband sent a photograph of one of their cats at home. “Waiting for you to come back,” he said.

“I think it’ll take 10 days, half a month,” she replied. “Take care of yourself.”

There is no known cure for Covid-19, the official name for the disease caused by the new coronavirus. So doctors rely on a cocktail of other medicines, mostly antiviral drugs, to alleviate the symptoms.

Ms. Deng’s doctor prescribed a regimen of arbidol, an antiviral medicine used to treat the flu in Russia and China; Tamiflu, another flu medicine more popular internationally; and Kaletra, an HIV medicine thought to block the replication of the virus. Ms. Deng was taking at least 12 pills a day, as well as traditional Chinese medicine.

Despite her optimism, she grew weaker. Her mother delivered home-cooked food outside the ward, but she had no appetite. To feed her, a nurse had to come at 8:30 each morning to hook her up to an intravenous drip with nutrients. Another drip pumped antibodies into her bloodstream, and still another antiviral medicine.

Dr. Xia, too, was severely ill, but appeared to be slowly fighting the infection. Her fever had subsided after a few days, and she began to breathe more easily after being attached to a ventilator.

Her spirits lifted. On Jan. 25, she told her colleagues she was recovering.

“ I will return to the team soon,” she texted them on WeChat.

“We need you the most,” one of her colleagues responded.

In early February, Dr. Xia asked her husband, Wu Shilei, also a doctor, whether he thought she could get off oxygen therapy soon.

“Take it easy. Don’t be too anxious,” he replied on WeChat. He told her that the ventilator could possibly be removed by the following week.

“I keep on thinking about getting better soon,” Dr. Xia responded.

There was reason to believe she was on the mend. After all, most coronavirus patients recover.

Later, Dr. Xia tested negative twice for the coronavirus. She told her mother she expected to be discharged on Feb. 8.

Decline: Days 4 to 15 After Hospitalization

By Ms. Deng’s fourth day in the hospital, she could no longer pretend to be cheerful. She was vomiting, having diarrhea and relentlessly shivering.

Her fever jumped to 101.3 degrees. Early in the morning on Feb. 5, she woke from a fitful sleep to find the medicine had done nothing to lower her temperature. She cried. She said she was classified as critically ill.

The next day, she threw up three times, until she was left spitting white bubbles. She felt she was hallucinating. She could not smell or taste, and her heart rate slowed to about 50 beats per minute.

On a phone call, Ms. Deng’s mother tried to reassure her that she was young and otherwise healthy, and that the virus would pass like a bad cold. But Ms. Deng feared otherwise. “I felt like I was walking on the edge of death,” she wrote in a social media post from her hospital bed the next day.

China defines a critically ill patient as someone with respiratory failure, shock or organ failure. Around 5 percent of infected patients became critical in China, according to one of the largest studies to date of coronavirus cases. Of those, 49 percent died. (Those rates may eventually change once more cases are examined around the world.)

While Dr. Xia appeared to be recovering, she was still terrified of dying. Testing can be faulty, and negative results don’t necessarily mean patients are in the clear.

She asked her mother for a promise: Could her parents look after her 2-year-old son if she didn’t make it?

Hoping to dispel her anxiety with humor, her mother, Jiang Wenyan, chided her: “He’s your own son. Don’t you want to raise him yourself?”

Dr. Xia also worried about her husband. Over video chat, she urged him to put on protective equipment at the hospital where he worked. “She said she would wait for me to return safely,” he said, “and go to the front line again with me when she recovered.”

Then came the call. Dr. Xia’s condition had suddenly deteriorated. In the early hours of Feb. 7, her husband rushed to the emergency room.

Her heart had stopped.

Recovery: Day 17 After Hospitalization

In most cases, the body repairs itself. The immune system produces enough antibodies to clear the virus, and the patient recovers.

By the end of Ms. Deng’s first week in the hospital, her fever had receded. She could eat the food her mother delivered. On Feb. 10, as her appetite returned, she looked up photos of meat skewers online and posted them wishfully to social media.

On Feb. 15, her throat swab came back negative for the virus. Three days later, she tested negative again. She could go home.

Ms. Deng met her mother briefly at the hospital’s entrance. Then, because Wuhan remained locked down, without taxis or public transportation, she walked home alone.

“I felt like a little bird,” she recalled. “My freedom had been returned to me.”

She had to isolate at home for 14 days. Her husband and daughter stayed with her parents.

At home, she threw out her clothing, which she had been wearing for her entire time in the hospital.

Since then, she has passed the time by playing with her cats and watching television. She jokes that she is getting an early taste of retirement. She does daily deep breathing exercises to strengthen her lungs, and her cough has faded.

The Chinese government has urged recovered patients to donate plasma, which experts say contains antibodies that could be used to treat the sick. Ms. Deng contacted a local blood bank soon after getting home.

She plans to go back to work as soon as the hospital allows it.

“It was the nation that saved me,” she said. “And I think I can pay it back to the nation.”

Death: Day 35 After Hospitalization

It was sometime after 3 a.m. on Feb. 7 when Dr. Xia was rushed to intensive care. Doctors first intubated her. Then, the president of the hospital frantically summoned several experts from around the city, including Dr. Peng Zhiyong, head of the department of critical care at Zhongnan Hospital.

They called every major hospital in Wuhan to borrow an extracorporeal membrane oxygenation, or Ecmo, machine to do the work of her heart and lungs.

Dr. Xia’s heart started beating again. But the infection in her lungs was too severe, and they failed. Her brain was starved of oxygen, causing irreversible damage. Soon, her kidneys shut down and doctors had to put her on round-the-clock dialysis.

“The brain acts as the control center,” Dr. Peng said. “She couldn’t command her other organs, so those organs would fail. It was only a matter of time.”

Dr. Xia slipped into a coma. She died on Feb. 23.

Dr. Peng remains baffled about why Dr. Xia died after she had seemed to improve. Her immune system, like that of many health workers, may have been compromised by constant exposure to sickness. Perhaps she suffered from what experts call a “cytokine storm,” in which the immune system’s reaction to a new virus engulfs the lungs with white blood cells and fluid. Perhaps she died because her organs were starved of oxygen.

Back at Dr. Xia’s home, her son, Jiabao — which means priceless treasure — still thinks his mother is working. When the phone rings, he tries to grab it from his grandmother’s hands, shouting: “Mama, mama.”

Her husband, Dr. Wu, doesn’t know what to tell Jiabao. He hasn’t come to terms with her death himself. They had met in medical school and were each other’s first loves. They had planned to grow old together.

“I loved her very much,” he said. “She’s gone now. I don’t know what to do in the future, I can only hold on.”

Produced by Rumsey Taylor and Josh Williams.

The country’s experience shows that steps to isolate the coronavirus and limit people’s movement need to be put in place early, with absolute clarity, then strictly enforced.

By Jason Horowitz, Emma Bubola and Elisabetta Povoledo

ROME — As Italy’s coronavirus infections ticked above 400 cases and deaths hit the double digits, the leader of the governing Democratic Party posted a picture of himself clinking glasses for “an aperitivo in Milan,” urging people “not to change our habits.”

That was on Feb. 27. Not 10 days later, as the toll hit 5,883 infections and 233 dead, the party boss, Nicola Zingaretti, posted a new video, this time informing Italy that he, too, had the virus.

Italy now has more than 53,000 recorded infections and more than 4,800 dead, and the rate of increase keeps growing, with more than half the cases and fatalities coming in the past week. On Saturday, officials reported 793 additional deaths, by far the largest single-day increase so far. Italy has surpassed China as the country with the highest death toll, becoming the epicenter of a shifting pandemic.

The government has sent in the army to enforce the lockdown in Lombardy, the northern region at the center of the outbreak, where bodies have piled up in churches. On Friday night, the authorities tightened the nationwide lockdown, closing parks, banning outdoor activities including walking or jogging far from home.

On Saturday night, Prime Minister Giuseppe Conte announced another drastic step in response to what he called the country’s most difficult crisis since the Second World War: Italy will close its factories and all production that is not absolutely essential, an enormous economic sacrifice intended to contain the virus and protect lives.

“The state is here,” he said in an effort to reassure the public.

But the tragedy of Italy now stands as a warning to its European neighbors and the United States, where the virus is coming with equal velocity. If Italy’s experience shows anything, it is that measures to isolate affected areas and limit the movement of the broader population need to be taken early, put in place with absolute clarity, then strictly enforced.

Despite now having some of the toughest measures in the world, Italian authorities fumbled many of those steps early in the contagion — when it most mattered as they sought to preserve basic civil liberties as well as the economy.

Italy’s piecemeal attempts to cut it off — isolating towns first, then regions, then shutting down the country in an intentionally porous lockdown — always lagged behind the virus’s lethal trajectory.

“Now we are running after it,” said Sandra Zampa, the under secretary at the Ministry of Health, who said Italy did the best it could given the information it had. “We closed gradually, as Europe is doing. France, Spain, Germany, the U.S. are doing the same. Every day you close a bit, you give up on a bit of normal life. Because the virus does not allow normal life.”

Some officials gave in to magical thinking, reluctant to make painful decisions sooner. All the while, the virus fed on that complacency.

Governments beyond Italy are now in danger of following the same path, repeating familiar mistakes and inviting similar calamity. And unlike Italy, which navigated uncharted territory for a Western democracy, other governments have less room for excuses.

Italian officials, for their part, have defended their response, emphasizing that the crisis is unprecedented in modern times. They assert that the government responded with speed and competence, immediately acting on the advice of its scientists and moving more swiftly on drastic, economically devastating measures than their European counterparts.

But tracing the record of their actions shows missed opportunities and critical missteps.

In the critical early days of the outbreak, Mr. Conte and other top officials sought to down play the threat, creating confusion and a false sense of security that allowed the virus to spread.

They blamed Italy’s high number of infections on aggressive testing of people without symptoms in the north, which they argued only created hysteria and tarnished the country’s image abroad.

Even once the Italian government considered a universal lockdown necessary to defeat the virus, it failed to communicate the threat powerfully enough to persuade Italians to abide by the rules, which seemed riddled with loopholes.

“It is not easy in a liberal democracy,” said Walter Ricciardi, a World Health Organization board member and a top adviser to the health ministry, who argued that the Italian government acted on the scientific evidence made available to it.

He said the Italian government had moved at a much faster clip, and took the threat much more seriously, than its European neighbors or the United States.

Still, he acknowledged that the health minister had struggled to persuade his government colleagues to move more quickly and that the difficulties of navigating Italy’s division of powers between Rome and the regions resulted in a fragmented chain of command and inconsistent messages.

“In times of war, like an epidemic,” that system presented grave problems, he said, adding that it perhaps delayed the imposing of restrictive measures.

“I would have done them 10 days before, that is the only difference.”

It Could Never Happen Here

For the coronavirus, 10 days can be a lifetime.

On Jan. 21, as top Chinese officials warned that those hiding virus cases “will be nailed on the pillar of shame for eternity,” Italy’s culture and tourism minister hosted a Chinese delegation for a concert at the National Academy of Santa Cecilia to inaugurate the year of Italy-China Culture and Tourism.

Michele Geraci, Italy’s former under secretary in the economic development ministry and a booster of closer relations with China, had a drink with other politicians but looked around uneasily.

“Are we sure we want to do this?” he said he asked them. “Should we be here today?”

With the benefit of hindsight, Italian officials say certainly not.

Ms. Zampa, the health ministry under secretary, said in retrospect she would have closed everything immediately. But in real time, it wasn’t that clear.

Politicians across the spectrum worried about the economy and feeding the country, and found it difficult to accept their impotence in the face of the virus.

Most importantly, Italy looked at the example of China, Ms. Zampa said, not as a practical warning, but as a “science fiction movie that had nothing to do with us.” And when the virus exploded, Europe, she said, “looked at us the same way we looked at China.”

But already in January, some officials on the right were urging Mr. Conte, their former ally and now political enemy, to quarantine schoolchildren in the northern regions who were returning from holidays in China, a measure aimed at protecting schools. Many of those children were from Chinese immigrant families.

Many liberals criticized the proposal as populist fear-mongering. Mr. Conte declined the proposal and responded that the northern governors should trust the judgment of education and health authorities who, he said, had proposed no such thing.

But Mr. Conte also demonstrated that he was taking the threat of contagion seriously. On Jan. 30, he blocked all flights in and out of China.

“We are the first country in Europe to adopt such a precautionary measure,” he said.

Over the next month, Italy responded swiftly to coronavirus scares. Two sick Chinese tourists and an Italian returning from China received care from a prominent infectious disease hospital in Rome. A false alarm led authorities to briefly confine passengers on a cruise ship docked outside of Rome.

‘Patient One,’ Super-spreader

When a 38-year-old man went to the emergency room at a hospital in Codogno, a small town in the Lodi province of Lombardy, with severe flu symptoms on Feb. 18, the case did not set off alarms.

The patient declined to be hospitalized and went home. He got sicker and returned to the hospital a few hours later and was admitted to a general medicine ward. On Feb. 20, he went into intensive care, where he tested positive for the virus.

The man, who became known as Patient One, had had a busy month. He attended at least three dinners, played soccer and ran with a team, all apparently while contagious and without heavy symptoms.

Mr. Ricciardi said Italy had the bad luck of having a super spreader in a densely populated and dynamic area who went to the hospital not once, but twice, infecting hundreds of people, including doctors and nurses.

“He was incredibly active,” Mr. Ricciardi said.

But he also had not had any direct contacts with China, and experts suspect he contracted the virus from another European, meaning Italy did not have an identifiable patient zero or a traceable source of contagion that could help it contain the virus.

The virus had already been active in Italy for weeks by that time, experts now say, passed by people without symptoms and often mistaken for a flu. It spread around Lombardy, the Italian region that has by far the most trade with China and the home of Milan, the country’s most culturally vibrant and business-centered city.

“Who we call ‘Patient One’ was probably ‘Patient 200,’ ” said Fabrizio Pregliasco, an epidemiologist.

On Sunday, Feb. 23, the number of infections clicked past 130 and Italy sealed off 11 towns with police and military checkpoints. The last days of Venice Carnival were canceled. The Lombardy region closed its schools, museums and movie theaters. The Milanese made a run on the supermarkets.

But while Mr. Conte again commended Italy for its firm hand, he also sought to downplay the contagion, attributing the high numbers of infected to Lombardy’s overzealous testing.

“We have been the first ones with the most rigorous and accurate controls,” he said on television, adding that more people in Italy appeared infected because “we did more tests.”

The next day, as infections surpassed 200, seven people died and the stock market plunged, Mr. Conte and his health aides doubled down.

He blamed the Codogno hospital for the spread, saying it had handled things in “a not-completely-proper way” and argued that Lombardy and Veneto, another northern region, were inflating the severity of the problem by diverging from global guidelines and testing people without symptoms.

As Lombardy officials scrambled to free up hospital beds, and the number of infected people rose to 309 with 11 dead, Mr. Conte said on Feb. 25 that “Italy is a safe country and probably safer than many others.”

On Friday, Mr. Conte’s office offered an interview on the condition that he could answer questions in writing. When sent questions, including those about his past statements, he declined to respond.

Mixed Messages Sow Confusion

Reassurances from leaders confused the Italian population.

On Feb. 27, Mr. Zingaretti posted his aperitivo picture. That same day, the country’s foreign minister, Luigi Di Maio, the former leader of one of the governing parties, the Five Star Movement, held a news conference in Rome.

“In Italy, we went from the risk of an epidemic to an infodemic,” Mr. Di Maio said, disparaging media coverage that highlighted the threat of the contagion, and adding that only “0.089 percent” of the Italian population was quarantined.

In Milan, only miles from the center of the outbreak, the mayor, Beppe Sala, publicized a ‘‘Milan Doesn’t Stop’’ campaign, and the Duomo, the city’s landmark cathedral that is a draw for tourists, reopened. People went out.

But on the sixth floor of the regional government headquarters in Milan, Giacomo Grasselli, who is the coordinator of the intensive care units throughout Lombardy, saw the numbers going up and quickly realized that it would be impossible to treat all the sick if the infections continued unabated.

His task force worked to match the sick to beds in intensive-care units in the nearest possible hospitals and appropriate dwindling resources.

At one of the daily meetings of about 20 health and political officials, he told the regional president, Attilio Fontana, about the growing numbers.

An epidemiologist showed the curves of infection. There was a catastrophe facing the region’s well-respected health system.

“We need to do something more,” Mr. Grasselli told the room.

Mr. Fontana, who had been pressing the central government for tougher action, agreed. He said that the mixed messages from Rome and the easing of restrictions had led Italians to believe “that everything was a joke, and they kept living as they used to.”

He said he appealed for tougher national measures in video conferences with the prime minister and other regional presidents, arguing that climbing numbers of cases threatened to collapse the hospital system in the north, but that his requests were repeatedly turned down.

“They were convinced that the situation was less serious and they did not want to hurt our economy too much,” said Mr. Fontana.

The government started providing some economic assistance, which would later be followed by a 25 billion euro ($28 billion) relief package, but the nation became divided between those who saw the threat and those who didn’t.

Ms. Zampa said that it was around that time that government learned that infections in the town of Vò, the virus epicenter of the Veneto region, had no epidemiological link to the Codogno outbreak.

She said that the health minister, Mr. Speranza, and Mr. Conte deliberated about what to do and within the day, they decided to close down much of the north.

In a surprise 2 a.m. news conference on March 8, when 7,375 people had already tested positive for coronavirus and 366 had died, Mr. Conte announced the extraordinary step of restricting movement for about a quarter of the Italian population in the northern regions that serve as the country’s economic engine.

“We are facing an emergency,” Mr. Conte said at the time. “A national emergency.”

A draft of the decree, leaked to Italian media on Saturday night, pushed many Milan residents to rush to the train station in crowds and attempt to leave the region, causing what many later considered a dangerous wave of contagion toward the south.

Yet the following day, most Italians were still confused about the severity of the restrictions.

To clarify the issue, the interior ministry issued “auto-certification” forms that would allow people to travel in and out of the locked-down area for work, health or “other” necessities.

In the meantime, some regional governors independently ordered people coming from the newly locked-down area to self quarantine. Others didn’t.

The broader restrictions in Lombardy also effectively lifted the quarantine on Codogno and other “red zone” towns linked to the original outbreak. Checkpoints disappeared. Local mayors complained that their sacrifices had been wasted.

A day later, on March 9, when the positive cases reached 9,172 and the death toll climbed to 463, Mr. Conte toughened the restrictions and extended them nationally.

But by then, some experts say, it was already too late.

Local Experiments

Italy is still paying the price of those early mixed messages by scientists and politicians. The people who have died in staggering numbers recently — more than 2,300 in the last four days — were mostly infected during the confusion of a week or two ago.

Roberto Burioni, a prominent virologist at the San Raffaele University in Milan, said that people had felt safe to go about their usual routines and he attributed the spike in cases last week to “that behavior.”

The government has urged national unity in obeying its restrictive measures. But on Saturday, hundreds of mayors from the hardest-hit areas told the government those measures were fatally insufficient.

Leaders in the north are desperate for the government to crack down harder.

On Friday, Mr. Fontana complained that the 114 troops the government deployed were insignificant, and that at least 1,000 should be sent. On Saturday, he closed public offices, work sites and banned jogging. He said in an interview that the government needed to stop messing around and “apply rigid measures.”

“My idea is that if we had shut everything in the beginning, for two weeks, probably now we would be celebrating victory,” he said.

His political ally, Luca Zaia, the president of the Veneto region, pre-empted the national government with his own crackdown, and said that Rome needed to enforce “a more drastic isolation,” including closing all stores and prohibiting public activities other than commuting to work.

“Walks should be banned,” he said.

Mr. Zaia has some credibility on the issue. As new infections have proliferated around the country, they have significantly dropped in Vò, a town of about 3,000 people that was one of the first quarantined and which had the country’s first coronavirus death.

Some government experts attributed that turnaround to the strict quarantine that had been in place for two weeks. But Mr. Zaia had also ordered blanket tests there, in defiance of international scientific guidelines and the national government. The government has argued that testing people without symptoms is a drain on resources.

“At least this slows down the virus’ speed,’’ Mr. Zaia said, arguing that testing helped identify potentially contagious people without symptoms. ‘‘And slowing down the virus’ speed allows the hospitals to breathe.’’

If not, the overwhelming number of patients would crater health care systems and cause a national catastrophe.

Americans and others, he said, “need to be ready.”

The largest travel restrictions in history to stop an outbreak haven’t been enough to head off a pandemic. We analyzed the movements of hundreds of millions of people to show why.

By Jin Wu, Weiyi Cai, Derek Watkins and James Glanz

Travel in China Before Lockdown

The timing of the outbreak could not have been worse. Hundreds of millions of people were about to travel back to their hometowns for Lunar New Year in late January.

1. Early transmission in December

Many of the first known cases clustered around a seafood market in Wuhan, a city of 11 million and a transportation hub. Four cases grew to dozens by the end of December. The true size of the outbreak was much larger even then — an invisible network of nearly 1,000 cases, or perhaps several times more.

2. Government’s slow response

With each patient infecting two or three others on average, even a perfect response may not have contained the spread. But Chinese officials did not alert the public to the risks right away. It wasn’t until Dec. 31 that they alerted the World Health Organization and released a statement — and a reassurance. “The disease is preventable and controllable,” the government said.

3. Outbreak seeded

On Jan. 21, Chinese officials finally acknowledged the risk of human-to-human transmission, but by then local outbreaks were already seeded in Beijing, Shanghai and other major cities.

Although the origin of the virus remains unclear, many early cases had links to a seafood market that sold exotic animals. The market is surrounded by large apartment buildings and a major train station — the perfect environment for an outbreak.

On Jan. 23, the authorities locked down Wuhan, and many cities followed in the next few weeks. Travel across China nearly stopped. But local outbreaks were already growing quickly.

4. Millions put under lockdown

From the end of January, Chinese cities started imposing a range of residential lockdowns on at least 760 million people — one in 10 people on earth.

After the first case was reported in Wuhan in December, the virus quickly spread to more than 300 cities in China, infecting at least 80,000 people.

As the outbreak took off in China in early January, international travel continued as normal. Thousands of people flew out of Wuhan, silently seeding outbreaks in cities all over the globe.

Busy International Travel Seeded Outbreaks

Air passengers from Wuhan went all over, based on normal travel patterns from before the outbreak.

1. Early overseas cases in mid-January

On Jan. 13, the first case outside China was reported in Bangkok. In the following days, Tokyo; Singapore; Seoul, South Korea; Ho Chi Minh City, Vietnam; and Hong Kong reported their first cases. The U.S. confirmed its first case near Seattle on Jan. 21.

2. Outbound travel stopped near end of January

It was only at the end of January that Wuhan was placed under a lockdown and airlines started canceling flights. By Jan. 31, when the United States announced it would shut down entry from China for non-Americans, travel out of Wuhan had basically stopped.

3. But it was too late

By Jan. 31, outbreaks were already growing in over 30 cities across 26 countries, most seeded by travelers from Wuhan.

As China started systematically testing, tracing and isolating patients, new cases there declined dramatically, showing that it was possible to slow the virus. Similar measures slowed the spread in Singapore, Hong Kong and South Korea. However, other countries have seen enormous growth. And the slowdowns in Asia could be reversed as soon as people return to work.

In the United States, where testing has lagged, President Trump suspended most travel from Europe on March 13. “The virus will not have a chance against us,” he said.

By March 16, more cases were reported outside mainland China than inside. The center of the pandemic shifted toward Europe and the United States.

The virus started spreading locally, moving easily in confined spaces like churches and restaurants, and infecting people who had not traveled to China — the start of a pandemic. By March, thousands of cases were reported in Italy, Iran and South Korea. China was no longer the main driver of the outbreak.

4. Local transmission

Public health officials made an enormous effort to isolate and track new cases. But cases continued to grow — and a new, concerning type of case was beginning to appear: infected people who had never traveled to China.

5. Cases with unknown sources

Even more alarming, cases began to appear that had no known link to previous infections.

6. Cases on six continents

By March, new cases had started to climb dramatically in countries like Italy and Iran. People traveling to those countries subsequently brought cases to countries as far away as Brazil. The virus has now spread to every continent except Antarctica.

But by then, the virus had a secure foothold. It continued to spread locally throughout parts of Seattle, New York City and across the country, once again outpacing efforts to stop it.

Photographs by Tyler Hicks

Written by Julie Turkewitz and Manuela Andreoni

Graphics by Jeremy White

The virus swept through the region like past plagues that have traveled the river with colonizers and corporations.

It spread with the dugout canoes carrying families from town to town, the fishing dinghies with rattling engines, the ferries moving goods for hundreds of miles, packed with passengers sleeping in hammocks, side by side, for days at a time.

The Amazon River is South America’s essential life source, a glittering superhighway that cuts through the continent. It is the central artery in a vast network of tributaries that sustains some 30 million people across eight countries, moving supplies, people and industry deep into forested regions often untouched by road.

But once again, in a painful echo of history, it is also bringing disease.

As the pandemic assails Brazil, overwhelming it with more than two million infections and more than 84,000 deaths — second only to the United States — the virus is taking an exceptionally high toll on the Amazon region and the people who have depended on its abundance for generations.

In Brazil, the six cities with the highest coronavirus exposure are all on the Amazon River, according to an expansive new study from Brazilian researchers that measured antibodies in the population.

The epidemic has spread so quickly and thoroughly along the river that in remote fishing and farming communities like Tefé, people have been as likely to get the virus as in New York City, home to one of the world’s worst outbreaks.

“It was all very fast,” said Isabel Delgado, 34, whose father, Felicindo, died of the virus shortly after falling ill in the small city of Coari. He had been born on the river, raised his family by it and built his life crafting furniture from the timber on its banks.

In the past four months, as the epidemic traveled from the biggest city in the Brazilian Amazon, Manaus, with its high-rises and factories, to tiny, seemingly isolated villages deep in the interior, the fragile health care system has buckled under the onslaught.

Cities and towns along the river have some of the highest deaths per capita in the country — often several times the national average. In Manaus, there were periods when every Covid ward was full and 100 people were dying a day, pushing the city to cut new burial grounds out of thick forest. Grave diggers lay rows of coffins in long trenches carved in the freshly turned earth.

Down the river, hammocks have become stretchers, carrying the sick from communities with no doctors to boat ambulances that careen through the water. In remote reaches of the river basin, medevac planes land in tiny airstrips sliced into the lush landscape only to find that their patients died while waiting for help.

The virus is exacting an especially high toll on Indigenous people, a parallel to the past. Since the 1500s, waves of explorers have traveled the river, seeking gold, land and converts — and later, rubber, a resource that helped fuel the Industrial Revolution, changing the world. But with them, these outsiders brought violence and diseases like smallpox and measles, killing millions and wiping out entire communities.

“This is a place that has generated so much wealth for others,” said Charles C. Mann, a journalist who has written extensively on the history of the Americas, “and look at what’s happening to it.”

Indigenous people have been roughly six times as likely to be infected with the coronavirus as white people, according to the Brazilian study, and are dying in far-flung river villages untouched by electricity.

Even in the best of times, the Amazon was among the most neglected parts of the country, a place where the hand of the government can feel distant, even nonexistent.

But the region’s ability to confront the virus has been further weakened under President Jair Bolsonaro, whose public dismissals of the epidemic have verged at times on mockery, even though he tested positive himself.

The virus has surged on his government’s disorganized and lackluster watch, tearing through the nation. From his first days in office, Mr. Bolsonaro has made it clear that protecting the welfare of Indigenous communities was not his priority, cutting their funding, whittling away at their protections and encouraging illegal encroachments into their territory.

To the outsider, the thickly forested region along the Amazon River appears impenetrable, disconnected from the rest of the world.

But that isolation is deceptive, said Tatiana Schor, a Brazilian geography professor who lives off one of the river’s tributaries.

“There is no such thing as isolated communities in the Amazon,” she said, “and the virus has shown that.”

The boats that nearly everyone relies on, sometimes crowded with more than 100 passengers for many days, are behind the spread of the virus, researchers say. And even as local governments have officially limited travel, people have continued to take to the water because almost everything — food, medicine, even the trip to the capital to pick up emergency aid — depends on the river.

Scholars have long referred to life on the Amazon as an “amphibious way of being.”

The crisis in the Brazilian Amazon began in Manaus, a city of 2.2 million that has risen out of the forest in a jarring eruption of concrete and glass, tapering at its edges to clusters of wooden homes perched on stilts, high above the water.

Manaus, the capital of Amazonas state, is now an industrial powerhouse, a major producer of motorcycles, with many foreign businesses. It is intimately connected to the rest of the world — its international airport sees about 250,000 passengers a month — and, through the river, to much of the Amazon region.

Manaus’s first documented case, confirmed on March 13, came from England. The patient had mild symptoms and quarantined at home, in a wealthier part of town, according to city health officials.

Soon, though, the virus seemed to be everywhere.

“We didn’t have any more beds — or even armchairs,” Dr. Álvaro Queiroz, 26, said of the days when his public hospital in Manaus was completely full. “People never stopped coming.”

Gertrude Ferreira Dos Santos lived on the city’s eastern edge, in a neighborhood pressed against the water. She used to say that her favorite thing in the world was to travel the river by boat. With the breeze on her face, she said, she felt free.

Then, in May, Ms. dos Santos, 54, fell ill. Days later, she called her children to her bed, making them promise to stick together. She seemed to know that she was about to die.

Eduany, 22, her youngest daughter, stayed with her that night. In early morning, as Eduany got up to take a break, her sister Elen, 28, begged her to come back.

Their mother had stopped breathing. The sisters, in desperation, attempted mouth-to-mouth resuscitation. At 6 a.m., the sun rising above the city, Ms. dos Santos died in their arms.

When men in white protective suits arrived later to carry away her body, the sisters began to wail.

Ms. dos Santos had been a single mother. Life had not always been easy. But she had maintained a sense of wonder, something her daughters admired. “In everything she did,” Elen said, “she was joyful.”

Her mother’s death certificate listed many underlying conditions, including longstanding breathing problems, according to the women. It also listed respiratory failure, a key indicator that a person has died of the coronavirus.

But her daughters didn’t believe she was a victim of the pandemic. She had certainly died of other causes, they said. God would not have given her such an ugly disease.

Along the river, people said similar things over and over, reluctant to admit to possible contagion, even as the health of their siblings and parents declined. Many seemed to think their families would be shunned, that a diagnosis would somehow tarnish an otherwise dignified life.

But as this stigma led people to play down symptoms of the virus out of fear, doctors said, the pandemic was spreading quickly.

After Manaus, the virus traveled east and west, racing away from the region’s health care center.

In Manacapuru, more than an hour from the capital, Messias Nascimento Farias, 40, carried his ailing wife to their car and sped down one of the region’s few country roads to meet the ambulance that could carry her to a hospital.

His wife, Sandra Machado Dutra, 36, gasped in his truck.

“The Lord is my shepherd, I shall not want,” he prayed over and over until he handed her to health care workers. They were lucky. She survived.

But for most people living along the river, hundreds of boat miles from Manaus, the fastest way to a major hospital is by plane.

Even before the virus arrived, people in far-flung communities with a life-threatening emergency could make a frantic call for an airplane ambulance that would take them to a hospital in the capital.

But the small planes turned out to be dangerous for people with Covid-19, sometimes causing blood oxygen levels to plummet as the aircraft rose. Very few of the airlift patients seemed to be surviving, doctors said.

Instead, physicians and nurses found themselves flying their patients to painful deaths far from everything and everyone they had loved.

One morning in May, a white plane touched down at the airport in Coari, about 230 miles from Manaus.

On the tarmac on a stretcher was Mr. Delgado, 68, the furniture maker, barefoot and barely breathing.

Dr. Daniel Sérgio Siqueira and a nurse, Walci Frank, exhausted after weeks of constant work, loaded him into the small cabin. As the plane rose, his oxygen levels began to dive.

Mr. Delgado’s daughter Isabel turned to the doctor in a panic. “My father is very strong,” she told him. “He is going to make it.”

When the Delgados finally reached the hospital in Manaus, Isabel was stunned by the scenes around her. Despairing relatives held up loved ones who had crumpled under the burden of disease, hurrying them in for treatment.

At the same time, patients who had managed to survive Covid-19 staggered out, into the jubilant arms of family and friends.

“I was just there,” she said, “praying that God would save my father.”

Mr. Delgado died a few days later. When Isabel found out, the doctor started crying with her.

She had no doubt that the river her father loved had also brought him the virus. Soon, she and five other family members fell ill, too.

When the coronavirus arrived in the Americas, there was widespread fear that it would take a devastating toll on Indigenous communities across the region.

In many places along the Amazon River, those fears appear to be coming true.

At least 570 Indigenous people in Brazil have died of the disease since March, according to an association that represents the country’s Indigenous people. The vast majority of those deaths were in places connected to the river.

More than 18,000 Indigenous people have been infected. Community leaders have reported entire villages confined to their hammocks, struggling to rise even to feed their children.

In many instances, the very health workers sent to help them have inadvertently spread the virus.

In the riverside hamlet of São José da Fortaleza, Chief Iakonero Apurinã’s relatives sent word, one by one, that they couldn’t eat, that they heard voices, that they were too sick to get up.

Soon, it seemed to the chief that everyone in her community was sick.

Chief Apurinã, 54, said her group of 35 Apurinã families had survived generations of violence and forced labor. They had arrived in São José da Fortaleza decades ago, believing that they would finally be safe.

It was the river, said the chief, that had sustained them, feeding, washing and cleansing them spiritually.

Then the new disease came, and the chief was ferrying traditional teas from home to home. Soon came her own cough and exhaustion. A test in Coari confirmed that she had caught the virus.

Chief Apurinã didn’t blame the river. She blamed the people who traveled it.

“The river to us is purification,” she said. “It’s the most beautiful thing there is.”

Miraculously, she said in mid-July, not a single person among the 35 families had died.

In Tefé, a city of 60,000 people nearly 400 miles along the river from Manaus, the virus had arrived with gale force.

At the small public hospital, where officials initially planned to accommodate 12 patients, nearly 50 crowded the makeshift Covid-19 unit. Dr. Laura Crivellari, 31, the hospital’s only infectious disease expert, took them in, doing what she could with two respirators, no intensive care unit, many sick colleagues — and no one to replace them.

At one of the worst moments, she was the only physician on duty for two days, overseeing dozens of critically ill patients.

The constant death pushed Dr. Crivellari to her breaking point. Some days she barely stopped to eat or drink.

At home, she shared her anguish with her partner. She was thinking of giving up medicine, she said. “I can’t carry on like this,” she told him.

The pandemic has been brutal on medical workers around the world, and it has been particularly difficult for the doctors and nurses navigating the vast distances, frequent communication cuts and deep supply scarcity along the Amazon.

Without proper training or equipment, many nurses and doctors along the river have died. Others have infected their families.

Dr. Crivellari knew her city was vulnerable. It’s a three-day boat ride from Manaus to Tefé, with ferries often carrying 150 people at a time.

“Our fear was that an infected person would contaminate the whole boat,” she said, “and that’s what ended up happening.”

By early July, the daily deaths in Tefé were dropping, and Dr. Crivellari began to celebrate the patients she had been able to save. She no longer thinks of quitting medicine.

Tefé, as a whole, took a cautious collective breath.

The virus, at least for the moment, had moved to a new place on the river.

João Castellano and Letícia Casado contributed reporting. Alain Delaquérière contributed research.