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Some Mississippi I.C.U.s are full, while Houston is on red alert.

The coronavirus continued its surge across the South and West, threatening to quickly overwhelm areas that had until recently been spared the worst of the pandemic. Some Mississippi intensive care units are at capacity as coronavirus cases climb and residents of Texas’s largest county received an emergency alert of the highest level, urging residents to remain at home, wear a mask and cancel gatherings. Houston’s mayor has said that I.C.U.s there were nearly at capacity. And at the University of Mississippi Medical Center, the I.C.U. is completely full, said Dr. Alan Jones, the assistant vice chancellor for clinical affairs.

“We’re very worried about the trajectory that we’re on and what’s going to happen in the next couple of weeks,” Dr. Jones said.

Mississippi already suffers from a lack of I.C.U. resources, particularly in rural communities, so urban hospitals like the University of Mississippi Medical Center were crowded even before the pandemic, Dr. Jones said. Now, doctors are coming up with non-traditional ways to create more space for intensive-care patients, like transforming the anesthesia recovery area into a makeshift I.C.U.

Dr. Thomas Dobbs, the state health officer, said at a news conference Thursday that the state had seen its highest number of hospitalizations in recent days.

Officials said that many state residents aren’t social distancing or wearing masks, and that they’re still coming together for large social gatherings.

In Texas, Harris County’s 4.7 million residents are now under the most severe coronavirus threat level, amid a steep rise in cases and official warnings that hospitals are nearing capacity.

An emergency alert was sent to the phones of Houston and other county residents on Friday evening, notifying them that the coronavirus threat level was bumped up to “LEVEL 1 (RED) SEVERE.” That means the outbreak is “severe and uncontrolled” and only getting worse, county officials said.

The county, by far Texas’s largest, has seen a sudden spike in reported cases in recent weeks. There have now been 28,255 cases in the county and 361 deaths, the majority of which have come within Houston’s city limits.

Gov. Greg Abbott has acknowledged that his reopening plan had gone awry. In an interview with KVIA-TV in El Paso on Friday evening, he said, “If I could go back and redo anything, it probably would have been to slow down the opening of bars.”

At a news conference the same day, Lina Hidalgo, the county’s elected leader, urged people to stay home unless it was absolutely necessary to leave and said that the situation was more dire than it had been in previous months, before Mr. Abbott’s statewide stay-at-home order began yielding on May 1 to a phased reopening.

“We are in a worse situation now than we were back then, and the only thing that worked back then was flattening that curve by staying home,” Ms. Hidalgo said, adding bluntly that “we opened too quickly.”

The city of Galena Park, which is also in Harris County, has announced a curfew from 10 p.m. to 5 a.m., starting Saturday “until further notice.”

For months, the world perilously dismissed evidence of silent spreaders.

In late January, a doctor in Munich discovered Germany’s first coronavirus case, but the diagnosis made no sense. The patient reported only one possible contact with the infection: a business colleague visiting from China who had seemed healthy during her stay.

The visitor later told colleagues that she had not started feeling ill until after the flight back to China. Days later, she tested positive for the coronavirus.

Although it is now widely accepted that seemingly healthy people can spread the virus, scientists at the time believed that only people with symptoms could infect others.

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

The possibility of transmission from seemingly healthy people could strongly limit the ability of public awareness campaigns, airport screenings and stay-home-if-you’re sick policies to stop the virus.

Dr. Rothe and her colleagues were among the first to warn the world. But interviews with doctors and public health officials in more than a dozen countries showed that for two crucial months, Western health officials and political leaders played down or denied the risk of symptomless spreading. Leading health agencies provided contradictory and sometimes misleading advice.

It is impossible to calculate the human toll of that two-month delay, but models suggest that earlier action might have saved tens of thousands of lives. 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.”

New outbreaks — and mixed messaging — leave some Americans questioning their shutdown sacrifices.

With new cases of the coronavirus suddenly surging across multiple states that had low and manageable caseloads just months ago, confusion and anger are swirling among those who obeyed lockdowns and drastic social measures out of a sense of civic duty to help bring the U.S. outbreak under control.

Nationwide, cases have risen 65 percent over the past two weeks. On Friday, the country reported more than 45,000 new infections, a record. By Saturday evening, more than 41,000 cases of the coronavirus had been announced across the United States, including single-day records in Florida, Nevada and South Carolina. It was the third consecutive day with more than 40,000 new cases in the country.

Before this week, the country’s largest daily total had been 36,738 on April 24.

Many business owners and workers who lost their jobs say they believe their leaders failed to prepare for the economic devastation that followed shutdowns that states had adopted, to differing degrees, since March. And they say that recent reopenings undercut their sacrifices.

In recent weeks, some conservatives said they had an additional concern: After weeks of being told that going to church, attending funerals and participating in protests was a willful, careless spurning of science, political leaders and some public health officials condoned — and even joined — the crowds protesting the killing of George Floyd and other black Americans.

“It’s just a real social whiplash,” said Philip Campbell, vice president of a pest control company in Central Michigan, who took part in the first protests against the lockdown in Lansing in April from the cab of his truck. “Two weeks ago you can’t go out because you are going to kill grandma. Now it’s ‘you have an obligation to go out.’ It leaves me feeling that the science and the public health authorities have been politicized.”

A number of states are reconsidering their reopening plans. Florida and Texas reimposed limits on bars, banning drinking inside or closing the premises entirely, as they scrambled to control what appeared to be a brewing public health catastrophe. All of this has left people with a bitterness toward public officials for what feels like a fumbling of their constituents’ sacrifices.

“Are we doing a full circle? Yes,” said Judy Ray, 57, a cosmetologist and hairdresser in Florida who was laid off from her job in March.

Some fault the state and city leaders who rushed to reopen while the virus surged in other corners of the country and who now face daunting outbreaks in their own backyards. Others fault a lack of federal leadership, and a White House that defies expert guidance.

At a news conference on Friday, Vice President Mike Pence deflected a question about whether President Trump was sending mixed messages by reiterating public health officials’ advice while resuming crowded campaign rallies and refusing to wear a mask.

“Even in a health crisis, the American people don’t forfeit our constitutional rights,” he said. “We were creating settings where people can choose to participate in the political process, and we will continue to do that.”

U.S. Roundup

In Florida and Texas, some residents wait over four hours to get tested.

Residents in Florida and Texas are facing long wait times and are being turned away at coronavirus testing sites that have reached capacity.

On Saturday, Stefano West drove more than an hour from Killeen, Texas, to Austin to find a testing site, noting that there weren’t many available closer to him. He said he then waited about four and a half hours in his car at the testing site in Austin, where officials spent anywhere from 10 to 45 minutes attending to each car.

“I was annoyed,” Mr. West said. “There wasn’t really communication. No one explained the process.”

In Houston, two stadium testing sites reached capacity just hours after opening on Saturday, according to the city’s health department.

Joi Ross-Moore, who lives in Houston, tried to get tested at a drive-through facility on Tuesday. She arrived at 8 a.m., she said, and was soon told that people had been waiting to get tested since 4 a.m., and that no more tests would be conducted that day.

At the Orange County Convention Center in Orlando, Fla., the first car found its spot in line for testing on Saturday at 12:30 a.m., according to the Florida Association of Public Information Officers. Testing didn’t start until 9 a.m. At a site in Jacksonville, Fla., the testing line was cut off in the early afternoon, before closing time, the association tweeted.

Florida on Saturday reported more than 9,500 new coronavirus cases, beating its record for new infections for the second consecutive day. The numbers have pushed the state’s total past 130,000 cases.

Mayor Carlos Giménez of Miami-Dade County said he would sign an emergency order closing beaches from July 3 to 7, citing the surge of cases and fears that mass gatherings over the Fourth of July holiday weekend could push the total even higher.

The order will ban gatherings of more than 50 people, including parades, and will also close parks to public fireworks displays — which the mayor said “must be viewed from one’s home or parked vehicle.”

“The closure may be extended if conditions do not improve,” Mr. Giménez said in a Friday news release.

Gov. Ron DeSantis said on Friday that drinking had been banned in bars because many businesses were not following social distancing restrictions.

“There was widespread noncompliance, and that led to issues,” Mr. DeSantis said at a news conference in Fort Myers.

In an apparent acknowledgment of the growing sense of anxiety over the surge in cases, Vice President Mike Pence postponed coming campaign stops in Arizona and Sarasota, Fla. Mr. Pence is still expected to make official visits in the coming days to Texas, Arizona and Florida to meet with governors and their health care teams, a senior administration official said.

In other national news:

• Officials in Nevada and South Carolina on Saturday reported their highest single-day case totals. At least 980 new cases were added in Nevada, more than double its previous daily high. In South Carolina, officials announced more than 1,600 new cases, nearly 300 more than the record set a day earlier.

• In Arizona, where a flood of new cases has overwhelmed testing centers, state health officials said Saturday that 87 percent of hospital I.C.U. beds were now in use. The state’s reopening has not been paused, but the governor recently allowed cities and counties to require masks to be worn in public.

• With coronavirus cases rising, officials in Washington announced on Saturday a pause for the eight counties that were eligible to move into phase four of the state’s reopening plans. Gov. Jay Inslee said the fourth phase “would mean a return to normal activity and we can’t do that now due to the continued rise in cases across the state.”

• A stunning proportion of coronavirus deaths in the United States — 43 percent — are linked to nursing homes and other long-term care facilities for older adults, according to a New York Times database. Infected people linked to nursing homes also die at a higher rate than the general population. As of June 26, the virus had infected more than 282,000 people at some 12,000 facilities.

A Starbucks barista gets $ 65,000 in donations after a customer complained about the mask policy.

A Starbucks barista in San Diego has more than $ 65,000 to fuel his dream of teaching dance after an online fund-raiser was created in response to a customer who was not wearing a mask complaining on Facebook about the company’s policies and her interaction with him.

The barista, Lenin Gutierrez, said in a Facebook video he posted on Wednesday that it had all started when he was working at the front register and asked the customer, Amber Lynn Gilles, if she had a face mask.

“No, I don’t need one,” Ms. Gilles told him, according to the video. She replied before he could show her a paper explaining the company’s mask policy during the coronavirus pandemic.

He said she started “cursing up a storm” and called people “sheep” before walking out. A few minutes later, she came back, he said, and asked for his name, took a photo of him and said she would call the corporate offices.

“I thought that was going to be the end of it,” said Mr. Gutierrez, who has worked at Starbucks since 2017. “I didn’t think it was going to come to this.”

Widely shared videos of customers arguing about not wearing masks at stores show how resistance is not uncommon and how businesses are trying to encourage compliance.

Starbucks said it had taken measures to keep employees and customers safe, including asking customers to wear facial coverings at its shops. A Starbucks representative said on Saturday that the company was abiding by guidance from the Centers for Disease Control and Prevention and local mandates. Since May 1, San Diego County has required face coverings in many public areas.

Cases in Egypt are soaring. Its mosques, cafes and restaurants are reopening anyway.

Egypt inched toward a semblance of normalcy on Saturday as mosques, cafes and restaurants reopened after three months, albeit under tight restrictions, even as the number of new coronavirus infections in the country continued to soar.

At dawn, masked worshipers flooded into Cairo’s ancient mosques, some over 1,000 years old, to say communal prayers for the first time in three months. Later, patrons slowly returned to the city’s coffees shops and restaurants, many with an evident sense of relief.

For months, business leaders have lobbied President Abdel Fattah el-Sisi to end the lockdown, arguing that the economic damage outweighed the health risks. On Friday, the International Monetary Fund approved a $ 5.2 billion loan to help Egypt get back on its feet, adding to an early promise of $ 2.8 billion at the start of pandemic.

Even so, tight limitations remain — restaurants are restricted to 25 percent capacity, water pipes are banned from cafes on sanitary grounds, and Friday prayers are still forbidden. The easing on Saturday had an anxious, tentative feel, with many Egyptians balancing their desire to socialize or worship together against an awareness that the disease was spreading fast.

“This has got to be a gradual process,” said Abdullah Mohammed, 33, a pharmacy worker sitting with friends by the Nile who had recently recovered from Covid-19.

Egypt’s health ministry has reported 62,755 infections and 2,620 deaths. With the infection rate touching new highs, doctors’ groups have warned of a crisis at overcrowded hospitals, where over 100 doctors have died for want of protective equipment and training.

Even so, Egypt’s airports are set to reopen on July 1.

Global RoundUP

The British police, facing repeated unauthorized gatherings, vow to crack down.

The police in Britain vowed to crack down on unauthorized gatherings this weekend after tens of thousands of people flocked to beaches, gathered outside for unauthorized parties and violated coronavirus restrictions in recent days.

In the country’s south, the police in several seaside cities issued dispersal orders after large numbers of beachgoers caused huge traffic jams and engaged in antisocial behavior this week — including brawls, excessive drinking and even defecating in public.

The police in London and other cities like Derby, in central England, vowed to robustly disperse any unauthorized gathering. Over 140 officers in London have been injured while patrolling protests and breaking up unsanctioned parties in recent weeks.

And in Liverpool, the police have been given the power to disperse any gathering of more than two people in the center of the city, after thousands of supporters flouted social distancing rules on Friday when celebrating their soccer club’s English Premier League championship title.

When fans turned out in large numbers for the second night in a row to celebrate Liverpool’s title — its first in 30 years — Mayor Joe Anderson denounced the presence of “too many people intoxicated and causing antisocial behavior.”

In other international news:

• More than 191,000 new infections were reported around the world on Friday, a single-day record. The total number of confirmed cases is nearing 10 million, and total deaths have passed 498,000.

• On Sunday, India reported 19,906 new cases, its highest one-day total. The country has had a total of 528,859 cases and 16,095 deaths.

• Venezuela is struggling to control its first major outbreak, amid fears that the virus will collapse the country’s dilapidated health care system and worsen its humanitarian crisis. Medical workers at hospitals in the second-largest city, Maracaibo, say they are overwhelmed with Covid-19 patients, and treatment is complicated by a lack of running water and protective equipment, and frequent power cuts.

• In Brazil, the pandemic is easing in big cities but gaining momentum in smaller towns. The Ministry of Health, in its latest epidemiological bulletin last week, reported that 65 percent of cases were concentrated outside state capitals and warned the virus had reached 88.6 percent of the country’s 5,570 cities.

  And Brazilian officials announced a deal with Oxford University and the pharmaceutical company AstraZeneca on Saturday that will see the nation producing a coronavirus vaccine that is one of about a dozen in the early stages of human testing. Fiocruz, a state-owned health research institute that has experience producing vaccines, will produce 100 million vaccines for Brazil, which has about 210 million residents.

• Serbia’s defense minister, Aleksandar Vulin, has tested positive for the coronavirus, shortly after returning from a visit to Moscow, where he had attended a huge and mostly face mask-free military parade in Red Square.

• On Sunday, voters in Poland will head to the polls in a tightly contested presidential election, the European Union’s first since the outbreak of the virus.

• Fears of a second wave of coronavirus infections in China have receded somewhat. As of Sunday morning, the authorities had reported 311 cases in an outbreak in Beijing, and linked cases in at least four provinces. But the recent surge confirmed fears of sporadic flare-ups even after countries tame their outbreaks.

• A randomized trial in Norway that aimed to test whether people who work out at gyms with modest restrictions are at greater risk of infection than those who do not has a tentative answer after two weeks: no. This week, in response to those findings, Norway reopened all of its gyms with the same safeguards in place that were used in the study. The trial, begun on May 22, included five gyms in Oslo with 3,764 members, ages 18 to 64, who had no underlying medical conditions. Researchers found just one coronavirus infection, in a person who had not used the gym before he was tested; it was traced to his workplace.

U.S. cases pass 2.5 million, but new C.D.C. data shows current testing vastly undercounts actual infections.

On Saturday, the number of coronavirus infections in the U.S. shot past 2.5 million, according to a New York Times database.

But the real number in many parts of the United States is more than 10 times higher than the reported rate, according to data released Friday by the Centers for Disease Control and Prevention.

The analysis is part of a wide-ranging set of surveys initiated by the C.D.C. to estimate how far the virus has spread. It found, for instance, that in South Florida as of April 10, under 2 percent of people had been exposed to the virus. (The proportion is likely to be much higher now, given the surge of infections in the state.) The C.D.C. estimated that 117,400 people in that region had been infected — about 11 times the reported number of 10,500 cases.

The results confirm what some scientists have warned about for months: that without wider testing, scores of infected people would go undetected and continue to circulate the virus.

“Our politicians can say our testing is awesome, but the fact is our testing is inadequate,” said Scott Hensley, a viral immunologist at the University of Pennsylvania who was not involved in the research.

The numbers indicate that even in areas hit hard by the virus, the overwhelming majority of people have not yet been infected, Dr. Hensley said.

“Many of us are sitting ducks who are still susceptible to second waves,” he said.

The difference between recorded infections and those that were missed was even more significant in Missouri, where about 2.65 percent of the population was infected with the virus as of April 26, although many people might not have felt sick. This number is about 24 times the reported rate: nearly 162,000 compared with the 6,800 believed to have been infected by then.

Dr. Robert R. Redfield, the director of the C.D.C., noted this trend on Thursday during a call with reporters.

“Our best estimate right now is for every case reported there were actually 10 other infections,” Dr. Redfield said.

The C.D.C. researchers tested samples from 11,933 people across six regions in the United States during discrete periods from March 23 through May 3: the Puget Sound region of Washington, where the first case in the country was diagnosed, as well as New York City, South Florida, Missouri, Utah and Connecticut.

As the virus jumps to new U.S. epicenters, travel restrictions follow.

When the coronavirus tore through states like New York, New Jersey and Louisiana during the first major wave of cases in April, governors in other regions were quick to place restrictions on travelers from states with early surges.

States like Florida and Arizona enacted laws requiring visitors from hot spots to quarantine for 14 days upon arrival — requirements that remain in place now, even as New York and New Jersey have sharply reduced their caseloads. In April, Gov. Greg Abbott of Texas authorized screening stations along the border with Louisiana, which have since been closed.

But as the virus has spread and caused immense surges across the Sun Belt, many governors who had previously enforced travel restrictions on visitors now find their own residents subject to similar measures elsewhere.

On Wednesday, the governors of New York, New Jersey and Connecticut agreed to impose quarantines on visitors from any states where infections crossed a certain threshold. At the time of the announcement, Gov. Andrew M. Cuomo of New York said that list would include Alabama, Arkansas, Arizona, Florida, North Carolina, South Carolina, Utah and Texas, but would change based on rolling averages.

Mr. Cuomo said anyone found disobeying the quarantine could face a fine of $ 2,000.

This week, a provision of an order by Mr. Cuomo went into effect that would make New York employees ineligible for paid sick leave if they voluntarily traveled to states with high infection rates.

“If we are going to maintain the progress we’ve seen, we need everyone to take personal responsibility,” Mr. Cuomo said in a statement about the policy.

Other states that have so far succeeded in keeping cases low and managing periodic flare-ups are drawing lines based on their own criteria.

As of July 1, Vermont will require travelers from any state outside the Northeast to complete either a 14-day quarantine or a seven-day quarantine followed by a negative test. To date, Vermont has kept positive cases to just 1,198, according to data compiled by The New York Times.

Some areas renew their focus on face masks as U.S. cases rise.

As many American cities report exponential case growth, leaders elsewhere are moving quickly to require face coverings in hopes of avoiding a similar fate.

“Let’s learn from Texas and Florida and what’s happening there now,” Dr. Rex Archer, the director of health in Kansas City, Mo., said Friday as his city moved to require masks inside businesses. “Their mitigations and closures weren’t as quickly adopted or embraced.”

In Anchorage, where case numbers are increasing but not exploding, Mayor Ethan Berkowitz said masks would be required at restaurants and stores next week. Taking that step now, he said, could limit the need for more drastic steps later.

“I do not want to go back to a hunker-down period,” Mr. Berkowitz said.

And in Little Rock, Ark., where cases have been ticking upward, Mayor Frank Scott Jr. cited alarming epidemiological forecasts in an order requiring masks in his city. Mr. Scott said he had recently tested negative for the virus.

“During the test results waiting period, my mind was centered on the times I inadvertently failed to where a mask, and whom could have been impacted,” Mr. Scott said on Twitter.

But Americans over all have received mixed messages from the start of the pandemic about the need for masks. The surgeon general in February tweeted a message encouraging Americans to “STOP BUYING MASKS! They are NOT effective in preventing general public from catching” the coronavirus. But the Centers for Disease Control and Prevention now recommends cloth face coverings “in public settings where other social distancing measures are difficult to maintain.”

Separately, U.S. officials last week warned against fraudulent postings, cards or flyers seen online claiming face mask exemptions under the Americans With Disabilities Act.

“Do not be fooled by the chicanery and misappropriation of the DOJ eagle,” said Matthew G.T. Martin, U.S. attorney for the Middle District of North Carolina. “These cards do not carry the force of law. The ‘Freedom to Breathe Agency,’ or ‘FTBA,’ is not a government agency.”

The pandemic poses a stark test for Obamacare.

The Affordable Care Act, the landmark health law that has been a subject of caustic debate for more than a decade, is being tested as never before as millions of Americans lose their jobs and medical coverage in the midst of the pandemic.

The law offers most newly unemployed people a path to stopgap health coverage, providing a cushion that did not exist during the last crushing recession, or ever before. But the crisis also highlights fundamental weaknesses in its patchwork system.

This week, as the United States reported several daily records in new coronavirus cases, the Trump administration continued the Republican Party’s push to abolish the law, with the Justice Department asking the Supreme Court to overturn the A.C.A.

Joseph R. Biden Jr., the presumptive Democratic nominee, supports improving and expanding the A.C.A., rather than replacing it with a “Medicare for all” system preferred by many in his party.

And as these political and legal battles play out, how the law functions in the coming months could sway both its durability and future.

“It’s not just a test — it’s a national study of what happens in states that implemented the A.C.A. as opposed to those that didn’t,” said Peter V. Lee, the executive director of Covered California, the state’s insurance marketplace created under the law.

Four out of every five people who have lost employer-provided health insurance during the pandemic are eligible for free coverage through expanded Medicaid programs or government-subsidized private insurance, according to the Kaiser Family Foundation, a nonpartisan research group. And many jobless 20-somethings have been able to join their parents’ plans. All three options were made possible by the law.

Yet nearly three million low-income people are ineligible for assistance in the 14 states that have declined to expand Medicaid under the law, including several where coronavirus cases are now spiking.

The U.S. must release children from migrant family detention centers, a judge rules.

Citing the severity of the pandemic, a federal judge in Los Angeles on Friday ordered the release of migrant children held in the country’s three family detention centers.

The order, which mandates their release by July 17, came after plaintiffs in a long-running case reported that some of the children had tested positive for the coronavirus. It applies to children who have been held for more than 20 days in detention centers run by Immigration and Customs Enforcement, including two in Texas and one in Pennsylvania.

There were 124 children living in those facilities on June 8, according to the ruling.

“The family residential centers are on fire, and there is no more time for half measures,” Judge Dolly M. Gee of the U.S. District Court for the Central District of California wrote in the order.

She also criticized the Trump administration for its spotty compliance with recommendations from the Centers for Disease Control and Prevention in the centers.

It was the first time a court had set a firm deadline for the release of minors in family detention if their parents designated a relative in the United States to take custody. Recent orders had required their “prompt” release.

Over all, about 2,500 immigrants in ICE detention have tested positive for the virus. The agency has said that it has released at least 900 people with underlying conditions and that it has shrunk the population in each facility to mitigate the spread of the virus.

Three long(-haired) months: barbershop before-and-afters.

The Times’s Claudio E. Cabrera was among the New Yorkers who rejoiced when the city’s salons and barbershops reopened this week. Here, he writes about the experience.

For the last three months, I’ve spent my Google Hangout work meetings wearing a Yankees cap.

Despite being a born-and-bred New Yorker, I am not a Yankees fan at all. But it was the only cap I had in my house to cover my lack of a shape-up, my lack of a haircut, with barbershops in the city closed because of the pandemic.

When people talk about the relationship between people of color and their barbers, they tend to forget that it’s not just that they raise your self-esteem and help you look good — they are people you can also share your life with, and who can share their life with you.

And they aren’t your typical friend. They don’t come out with you to the bar. You may never go on a guys’ trip with them. You have those friends.

But your barber is your part-time therapist, and sometimes you are his.

In a parallel to H.I.V., studies find the coronavirus can deplete vital immune cells.

The coronavirus initially seemed to be another respiratory illness. But it has turned out to affect not just the lungs, but also the kidneys, heart and circulatory system — even, somehow, the senses of smell and taste.

Now, researchers have discovered yet another unpleasant surprise. In many patients hospitalized with the coronavirus, the immune system is threatened by a depletion of certain essential cells, suggesting eerie parallels with H.I.V.

One of the more detailed studies, published as a preprint and under review at Nature Medicine, was conducted by Dr. Adrian Hayday, an immunologist at King’s College London.

He and his colleagues compared 63 Covid-19 patients with 55 healthy people, some of whom had recovered from coronavirus infections.

One of the most striking aberrations in Covid-19 patients, the researchers found, was a marked increase in levels of a molecule that sends T cells to areas of the body where they are needed.

The result: a confused response from the immune system.

Some experts have wondered whether antiviral treatment makes sense for severely ill Covid-19 patients, if their main affliction is an immune system overreaction. But if the virus directly causes the immune system to malfunction, Dr. Hayday said, then an antiviral makes sense.

Tips for beginner (or returning) bicyclists

For anyone eager to get out on the road on a bicycle, here are a few things to remember.

Reporting was contributed by Reed Abelson, Manuela Andreoni, Matt Apuzzo, Nicholas Bogel-Burroughs, Julie Bosman, Claudio E. Cabrera, Tess Felder, Selam Gebrekidan, Manny Fernandez, Abby Goodnough, Andrew Higgins, Shawn Hubler, Miriam Jordan, Louis Keene, David D. Kirkpatrick, Gina Kolata, Anatoly Kurmanaev, Apoorva Mandivilli, Patricia Mazzei, Giulia McDonnell Nieto del Rio, Sarah Mervosh, Zach Montague, Dave Montgomery, Christina Morales, Elian Peltier, Brad Plumer, Frances Robles, Somini Sengupta, Michael D. Shear, Jeanna Smialek, Mitch Smith, Sheryl Gay Stolberg, Sabrina Tavernise, Jim Tankersley, Declan Walsh and Vivian Wang.