The White House sets conditions on testifying before Congress as the virus’s partisan divide widens.

Administration officials will only testify before Congress if committee leaders agree to conduct the hearings in person, the White House informed Congress on Friday. The decision amounted to a direct challenge to new House rules that allow committees and lawmakers to conduct their work remotely during the coronavirus pandemic, but it was also symbolic of a growing partisan divide about how to conduct political business in an era of concerns about public health.

The new condition, outlined in a notice obtained by The New York Times, is in addition to a policy the administration instituted this spring barring administration and agency officials from testifying without the express permission of Mark Meadows, the White House chief of staff.

“The Administration is willing to make accommodations, but only when Congress is similarly willing to make accommodations, including agreeing to appear in person,” the White House said, according to a notice sent to congressional staff members. The notice acknowledged exceptions could be made in instances in which a witness needed to quarantine.

The policy comes as the House of Representatives plans to pull back from its usual activities. Democratic leaders announced on Friday that they expect to call the chamber into session for votes for only three weeks over the next three months — a substantially scaled-back summer schedule.

President Trump, in contrast, is ramping up his campaigning in the coming weeks. While he still has no mass rallies scheduled, he will resume in-person fund-raisers next month under new restrictions, according to Republican Party officials.

Mr. Trump will headline a June 11 fund-raiser at a private home in Dallas, and a June 13 fund-raising event at his golf club in Bedminster, N.J.

The Dallas event will cost $ 580,600 per couple to attend, a party official said, and the Bedminster event will be held outdoors and cost $ 250,000 per person.

Only about 25 attendees are expected at each of the events, a nod to social distancing recommendations. But each attendee will have to submit to a virus test, complete a wellness questionnaire, and pass a temperature screening.

Vice President Mike Pence is also expected to resume attending fund-raisers in June, but the future of the Republican convention scheduled for August in Charlotte, N.C., remained in limbo as Republicans planning the event traded demands with North Carolina’s Democratic governor.

In a joint letter to Gov. Roy Cooper on Thursday, the Republican National Committee chairwoman, Ronna McDaniel, and the president of the convention committee, Marcia Lee Kelly, laid out a deadline of June 3 to approve safety measures to prevent the spread of the virus during the event.

On Friday, Mr. Cooper’s administration responded with a letter of its own, asking Ms. McDaniel and Ms. Kelly to detail the R.N.C.’s plans to protect participants, and to specify whether the president was still set on holding a large-scale nominating event where crowds would not wear face masks.

Inside an extraordinary G.O.P. event in South Carolina, where there were few masks and no social distancing.

The first mention of the coronavirus pandemic was a joke.

A master of ceremonies was explaining to a crowd of more than 100 people why the keynote speakers — home-state Senators Lindsey Graham and Tim Scott — were running a bit late.

“You have to understand, they haven’t got to do any politicking for a while,” said the M.C., Robert Rabon of the Horry County Republican Party. “They’re like a kid in a candy store — they’re going wild!”

And with that passing mention, South Carolina Republicans returned to the normal rhythm of the campaign trail, coronavirus all the same.

The outdoor gathering here on Thursday was a send-off event for Cleo Steele, a longtime Republican Party operative in Horry County, who is retiring to Ohio. Speakers shared the same microphone. Local and state political candidates greeted voters with handshakes and squeezed tight for pictures. Of all the people gathered outside the county Republican office — many of them senior citizens — fewer than a dozen wore masks.

“Social distancing guidelines are recommended,” the invitation had said. “Hand sanitizer and face masks will be available.” In reality, according to interviews with more than a dozen attendees, the event was an active rejection of behavior that the hyper-conservative crowd has come to associate with liberal enemies in recent months — wearing masks and gloves, staying six feet away from other people, avoiding physical touch.

To treat the coronavirus as something to be feared, they said, was a political act incongruous with their values.

Dwayne “Duke” Buckner, who is challenging Mr. Graham in the Republican Senate primary next month and came to the event to meet voters, said he had recently stopped adhering to public health guidelines, which he described as overly burdensome.

“You can quote me on this,” Mr. Buckner said. “When the good Lord calls you home, a mask ain’t going to stop it.”

New York City moves toward reopening.

Gov. Andrew M. Cuomo of New York said on Friday that he expected the city to meet several benchmarks that would permit millions of virus-weary residents to enjoy the first signs of a normal life as early as June 8. Retail stores could open for curbside or in-store pickup, and nonessential construction and manufacturing could resume, part of an initial phase that could send as many as 400,000 people back to work.

As other parts of the nation, including less populated sections of New York State, have already reopened, New York City, which lost more than 20,000 lives to the virus, has taken much longer to recover.

Deaths in New York have dropped to only dozens a day, rather than the 700 or 800 a day that were taking place in April.

That progress largely came because many New Yorkers followed the rules, and have been wearing masks and maintaining social distance as requested. The rewards of vigilance have been manifest not only in decreasing fatalities, but also in the declining number of people testing positive for the virus and those requiring hospital stays because of it.

“I am proud of the way New York is figuring it out,” Mr. Cuomo said.

But even with the strides the city has made, the road to normalcy will no doubt be steep and rocky. Since February, nearly 900,000 local jobs have vanished and thousands of businesses have closed their doors — some forever. Revenues from sales taxes are expected to drop by $ 1 billion, part of a frightening $ 9 billion estimated budget shortfall that could push officials into risky borrowing and force drastic cuts to essential city services.

Washington and Los Angeles are among the other major cities easing restrictions.

Major cities like Washington, D.C., and Los Angeles were set to continue easing restrictions, with Washington reopening outdoor seating at restaurants. The number of diners in a party will be limited to six people and tables will have to be spaced at least six feet apart. Hair salons and barbershops were also permitted to open for appointment-only haircuts with stations six feet apart. The city’s parks are open, but not its playgrounds.

The northern suburbs of Virginia also began limited reopenings on Friday, while suburban counties in Maryland remain shut. Earlier this week, Dr. Deborah L. Birx, the White House virus response coordinator, singled out the Washington region as among a handful of metropolitan areas where positive test rates remained high.

In California, officials in hard-hit Los Angeles announced plans to lift some restrictions. Restaurants and hair salons will be able to reopen as soon as this weekend if they meet Los Angeles County’s guidelines and safety protocols. As of Friday, 48 out of 58 California counties had met the readiness criteria for reopening. Each county is required to file documentation showing that it qualifies for variance from the stay-at-home order.

Illinois is expected to enter its third phase of reopening in the coming days, bringing barbershops, salons, retail stores and other businesses back with some limitations. Gatherings of more than 10 people remain banned. New cases in the state continue to mount, with an average of more than 1,800 new cases a day over the past week.

In the next few days, Ohio will allow child care centers, day camps and some catering and banquet businesses to reopen. Most businesses there have reopened over the last month.

The governor of Connecticut announced Friday that his state would begin to allow gatherings of up to 10 people indoors and 25 people outdoors. Houses of worship would be allowed to hold indoor gatherings at 25 percent capacity or up to 100 people, whichever is fewer; outdoor services could include up to 150 people as long as social distancing was observed. Casinos on land held by sovereign nations could also reopen, a move the governor, Ned Lamont, had opposed.

In New Jersey, Gov. Phil Murphy said that over the next several weeks the state would permit child care services to fully open and some summer programs for children to begin operating.

“As more and more workers prepare to get back out to their jobs, we must ensure a continuum of care for their children,” he said.

Trump says the U.S. will end its relationship with the W.H.O., alarming health experts.

After spending weeks accusing the World Health Organization of helping the Chinese government cover up the early days of the coronavirus epidemic in China, President Trump said on Friday that the United States would terminate its relationship with the agency.

“The world is now suffering as a result of the malfeasance of the Chinese government,” Mr. Trump said in a speech in the Rose Garden. “Countless lives have been taken, and profound economic hardship has been inflicted all around the globe.”

In his 10-minute address, Mr. Trump took no responsibility for the deaths of 100,000 Americans from the virus, instead saying China had “instigated a global pandemic.”

There is no evidence that the W.H.O. or the government in Beijing hid the extent of the epidemic in China, and public health experts generally view Mr. Trump’s charges as a way to deflect attention from his administration’s own bungled attempts to respond to the virus’s spread in the United States.

A spokeswoman for the W.H.O. in Geneva, where word of Mr. Trump’s announcement arrived around 9 p.m., said the agency would not have a response until Saturday.

Public health experts in the United States reacted to Mr. Trump’s announcement with alarm.

“We helped create the W.H.O.,” said Dr. Thomas Frieden, the former director of the Centers for Disease Control and Prevention, which has worked with the organization since its creation in 1948.

“We’re part of it — it is part of the world,” Dr. Frieden said. “Turning our back on the W.H.O. makes us and the world less safe.”

It’s not whether you were exposed to the virus. It’s how much.

When experts recommend wearing masks, staying at least six feet away from others, washing your hands frequently and avoiding crowded spaces, what they’re really saying is: Try to minimize the amount of virus you encounter.

A few viral particles cannot make you sick — the immune system would vanquish the intruders before they could. But how much virus is needed for an infection to take root? What is the minimum effective dose?

A precise answer is impossible, because it’s difficult to capture the moment of infection. Scientists are studying ferrets, hamsters and mice for clues but, of course, it wouldn’t be ethical for scientists to expose people to different doses of the virus, as they do with milder cold viruses.

“The truth is, we really just don’t know,” said Angela Rasmussen, a virologist at Columbia University in New York. “I don’t think we can make anything better than an educated guess.”

So-called super-spreaders seem to be particularly gifted in transmitting the virus, although it’s unclear whether that’s because of their biology or their behavior.

On the receiving end, the shape of a person’s nostrils and the amount of nose hair and mucus present — as well as the distribution of certain cellular receptors in the airway that the virus needs to latch on to — can all influence how much virus it takes to become infected.

The crucial dose may also vary depending on whether it’s ingested or inhaled.

But the virus’s uncertainties will weigh on American employers as they contemplate sweeping new recommendations from the C.D.C. on the safest ways to reopen their offices.

The recommendations include temperature and symptom checks for arriving employees; keeping desks six feet apart; and the wearing of face coverings at all times.

If followed, the guidelines would lead to a far-reaching remaking of the corporate work experience. They even upend years of advice on commuting, urging people to drive to work by themselves, instead of taking mass transportation or car-pooling, to avoid potential exposure to the virus.

Stung by the loss of major events, TV networks brace for billions in lost ad revenue.

For the TV networks, 2020 was supposed to be a big year. The presidential election and Tokyo Olympics would keep people watching, and companies were expected to spend more than usual on commercial time.

But with the Summer Games postponed and campaign rallies on lockdown, television advertising revenue is likely to drop 12 percent this year, according to a projection by the research firm MoffettNathanson. Networks will lose out on $ 25.5 billion in spending, according to a report released on Thursday by the WARC research group.

Viewership is not the problem; now that millions of homebound people have limited entertainment options, many people are watching more TV. But the economic fallout of the pandemic has caused companies to slash their advertising budgets by more than 40 percent, according to the research firm Kantar. To hedge against that drop in revenue, networks have offered commercial time at double-digit discounts, and some companies have turned to bare-bones advertisements instead of more elaborately produced commercials.

Before the pandemic, 30 percent of ad spending in the United States went toward TV commercials, while 56 percent went to purely digital platforms. By the end of the year, “this gap is really going to blow out,” with TV “falling more dramatically,” said Michael Nathanson, a founding partner of the MoffettNathanson research firm.

Another senator, Bob Casey of Pennsylvania, tests positive for virus antibodies.

Senator Bob Casey, Democrat of Pennsylvania, disclosed Friday that he had tested positive for coronavirus antibodies, and said that he planned to donate his plasma to help fight the virus.

Mr. Casey was the second senator in two days to announce a recent positive antibody test, after Tim Kaine of Virginia said Thursday he had the antibodies, and the third now known to have contracted Covid-19. Senator Rand Paul, Republican of Kentucky, tested positive for the virus earlier this spring, and continued to come into close contact with colleagues in the Capitol in the days before receiving his results.

It was unclear from his statement whether Mr. Casey could have been carrying the virus when Congress was in session, at a time when it was known to be infecting others on Capitol Hill.

Mr. Casey said he had experienced “mild” Covid-19 symptoms including “for a number of days” this spring, without providing a precise timetable. He said he had been able to work remotely, in isolation, through the symptoms and that by mid-April he was feeling better. Because the case was mild, Mr. Casey said his doctor at the time recommended against getting a coronavirus test.

More recently, Congress’s attending physician advised Mr. Casey to take an antibody test.

“The results revealed substantial levels of Covid-19 antibody in my blood, significantly more than the amount required to qualify me as a plasma donor,” he said on Friday. “In an effort to help others fighting this virus, I will be making my first donation today.”

The economy’s future is a question mark.

There is widespread agreement that the United States economy will soon begin to recover from coronavirus lockdowns. The big debate is whether that rebound will resemble a V, a W, an L or a Nike Swoosh.

Increasingly, economists and analysts are penciling in another glyph: a question mark.

Forecasters often label their expectations for a post-recession rebound with letters — a “V” suggests a rapid recovery, a “W” a double-dip, and so on — but that’s hard to do this time around. As all 50 states begin to open up, consumers trickle out of their homes and businesses make adjustments — crafting everything from restaurant protocols to remote auctions — the path ahead is wildly uncertain, making prognostication dicey.

It isn’t just Wall Street forecasters eschewing declarative forecasts and the alphabet in favor of a range of what-ifs. From the Federal Reserve to the White House, analysts have suggested that posing confident prognostications is probably more misleading than helpful.

Larry Kudlow, director of the White House National Economic Council, said at an event sponsored by The Washington Post that he shares President Trump’s expectation for a rapid bounce-back, but suggested that there are wide ranges around those estimates.

“It’s really hard to model a virus, or a pandemic, the likes of which we haven’t seen in 100 years,” Mr. Kudlow said.

“You can have your own Vs; there’s Vs, there are lesser Vs,” Mr. Kudlow said. “There are combos of Us and Vs.”

Cognizant of that uncertainty, the White House confirmed it will not even issue an update to its economic forecasts this summer, breaking decades of tradition.

These athletes had the coronavirus. Will they ever be the same?

Athletes tend to view themselves as perhaps better equipped than the general population to avoid the worst consequences of the disease.

Yet interviews with athletes who have contracted it — from professionals to college athletes to weekend hobbyists — revealed their surprise at the potency of its symptoms, struggles to reestablish workout regimens, lingering battles with lung issues and muscle weakness, and unsettling bouts of anxiety about whether they would be able to match their physical peaks.

“It definitely shook me up a bit — it was very surreal, you know?” Von Miller, a linebacker for the Denver Broncos who contracted the virus, said in an interview.

Panagis Galiatsatos, a pulmonary physician and assistant professor at Johns Hopkins, singled out three complications from Covid-19 that could be of particular concern to athletes.

First, coronavirus patients, like anyone with a serious respiratory infection, were at risk for long-term lung issues. He also considered the high incidence of blood clots seen as potentially troubling for athletes because people diagnosed with blood clots, and prescribed blood thinners, are typically discouraged from participating in contact sports.

Finally, Dr. Galiatsatos said patients placed on ventilators and confined to a bed often lost between 2 and 10 percent of their muscle mass per day.

Large crowds turn out across the country to protest George Floyd’s death.

As restrictions eased across the country, new measures were issued and under consideration in some cities and states where protests were growing over the killing of George Floyd, a black man who died in Minneapolis after a white police officer held his knee on Mr. Floyd’s neck for about nine minutes.

The Minnesota authorities said Friday afternoon that the officer, Derek Chauvin, who was fired after the episode, had been arrested and charged with third-degree murder.

Earlier, large crowds of demonstrators had gathered against the backdrop of a pandemic that had kept many residents from engaging with one another directly for months. Last week, Dr. Deborah L. Birx, the White House virus response coordinator, said that Minneapolis was considered a hot spot for virus cases.

In New York, where the city is still under restrictions to stem the spread of the virus, hundreds of people joined the protests on Thursday, and more than 40 were arrested.

Scientists are questioning the validity of an influential hydroxychloroquine study.

More than 100 scientists and clinicians have questioned the authenticity of a huge hospital database that was the basis for an influential study published last week that concluded that treating people who have Covid-19 with chloroquine and hydroxychloroquine did not help and might have increased the risk of abnormal heart rhythms and death.

In an open letter to the editor of The Lancet, which published the study’s findings, and the paper’s authors, the scientists asked the journal to provide details about the provenance of the data and called for the study to be independently validated by the World Health Organization or another institution.

A spokeswoman for Dr. Mandeep R. Mehra, the Harvard professor who was the paper’s lead author, said on Friday that the study’s authors had asked for an independent academic review and audit of their work.

Use of the malaria drugs chloroquine and hydroxychloroquine to prevent and treat Covid-19 has been a focus of intense public attention. President Trump has promoted the promise of hydroxychloroquine, despite the absence of gold-standard evidence from randomized clinical trials to prove its effectiveness, and recently said he was taking it himself in hopes of preventing coronavirus infection.

Fed chair says ‘red lines’ were crossed to help economy, and experts say the path to recovery remains uncertain.

Jerome H. Powell, the chair of the Federal Reserve, said central bankers had seen the need to use their tools “to their fullest extent” as coronavirus restrictions shuttered economies around the globe and caused United States unemployment to soar.

“We crossed a lot of red lines, that had not been crossed before,” Mr. Powell said in a webinar on Friday. He added that he was comfortable with what the Fed had done, because “this is that situation in which you do that, and you figure it out afterward.”

The Fed cut interest rates to near-zero and rolled out unlimited bond purchases to soothe markets, while setting up emergency lending programs to keep credit flowing to businesses and state governments. Several of those tiptoe into uncharted territory for the central bank, including programs to buy corporate bonds and purchase debt from states and large cities.

But even with that extraordinary support, the Fed chair made it clear that there is uncertainty about what will happen next, acknowledging that “a full recovery of the economy will really depend on people being confident that it’s safe to go out.”

Economists say that the path ahead for the economy is wildly uncertain, as massive questions loom over whether additional virus outbreaks will shutter the economy again, and over whether and when consumers will come back to stores. While a quick “V”-shaped recovery seems unlikely — and an “L” in which growth fails to rebound also seems off the table — what path the rebound will follow is a question mark.

“We’re all prefacing what we say with: We’re not epidemiologists,” said Jim O’Sullivan, chief U.S. macro strategist at TD Securities.

Global updates: Countries are opening up, even as their caseloads rise.

There have been nearly 700,000 new known cases reported in the past week as the virus spreads in Latin America. But many countries are sputtering into reopenings at what experts fear may be the worst time.

Here’s what you need to know about flying.

Airplane travel is inching back, but staying safe remains a concern and safety protocols are changing. We have tips for approaching your next flight.

Reporting was contributed by Mike Baker, Karen Barrow, Alan Blinder, Julie Bosman, Ben Casselman, Emily Cochrane, Andrew Das, Nicholas Fandos, Dana Goldstein, Jenny Gross, Maggie Haberman, Astead W. Herndon, Andrew Jacobs, Zolan Kanno-Youngs, Annie Karni, Andrew Keh, Apoorva Mandavilli, Donald G. McNeil, Jr., Andy Newman, Sona Patel, Roni Caryn Rabin, Matt Richtel, Katie Rogers, Dagny Salas, Marc Santora and Eileen Sullivan.