America’s most populous state is ordering its residents to stay at home.

Gov. Gavin Newsom of California on Thursday ordered Californians — all 40 million of them — to stay at home as much as possible in the coming weeks as the state confronts the escalating coronavirus outbreak. The order represents the most drastic measure any governor has taken to control the virus, and a decision that Governor Andrew Cuomo of New York, which has far more cases than in California, has resisted taking.

Mr. Newsom made the announcement from the state’s emergency operations center in Sacramento, normally a place where emergency workers coordinate responses to wildfires and earthquake, and spoke in stark terms of the risk the virus poses to the population.

Citing a model that state planners have been using, suggesting that 56 percent of Californians, or more than 25 million people, could be infected over eight weeks, Mr. Newsom said, “I think it’s time I tell you what I tell my family.”

“This is not a permanent state, this is a moment in time,” he said. “We will look back at these decisions as pivotal.”

Earlier in the week several counties in the Bay Area, plus Sacramento, issued orders that residents essentially shelter in place, although there are several exceptions — which also apply to the state order — such as going to buy groceries or picking up prescriptions.

The governor’s order came as cases in the United States surged past 10,000 on Thursday, prompting sweeping action from several other state leaders who had previously been reluctant to order imposing changes to daily life.

In Texas, Gov. Greg Abbott declared a public health disaster for the first time since 1901 and issued an executive order stopping dine-in service at restaurants and bars. The order also called for closing schools, and a ban on gatherings of more than 10 people statewide.

Florida’s southernmost county, which includes the Keys, ordered on Thursday all of its hotel to close. The move, at the height of the state’s tourist season, is expected to deal a severe blow to the local economy.

A global arms race for a coronavirus vaccine is underway.

In the three months since the virus began its deadly spread, China, Europe and the United States have all set off at a sprint to become the first to produce a vaccine. But while there is cooperation on many levels — including among companies that are ordinarily fierce competitors — hanging over the effort is the shadow of a nationalistic opportunity for the winner to potentially gain the upper hand in dealing with the economic and geostrategic fallout from the crisis.

What began as a question of who would get the scientific accolades, the patents and ultimately the revenues from a successful vaccine is suddenly a broader issue of urgent national security. And behind the scramble is a harsh reality: Any new vaccine that proves potent against the coronavirus — clinical trials are underway in the United States, China and Europe already — is sure to be in short supply as governments try to ensure that their own people are the first in line.

In China, 1,000 scientists are at work on a vaccine, and the issue has already been militarized: Researchers affiliated with the Academy of Military Medical Sciences have developed what is considered the nation’s front-runner candidate for success and is recruiting volunteers for clinical trials.

President Trump has talked with pharmaceutical executives about making sure a vaccine is produced on American soil, to assure the United States controls its supplies. German government officials said they believed he tried to lure a German company, CureVac, to do its research and production, if it comes to that, in the United States.

The company has denied it received a takeover offer, but its lead investor made clear there was some kind of approach.

The epidemic is now bigger in Europe, where governments aren’t used to giving harsh orders, and citizens aren’t used to following them.

The macabre milestones keep coming. By Wednesday, Europe had recorded more coronavirus cases and fatalities than China. On Thursday, Italy — by itself — passed China in reported deaths.

While China claims to have lowered its rate of new cases essentially to zero, Europe’s numbers grow faster each day — about 100,000 confirmed infections and 5,000 deaths in all so far — suggesting that the worst is yet to come.

So how is it that the new disease, Covid-19, has hit harder in Europe, which had weeks of warning that the epidemic was coming, than in China, where the virus originated and where there are twice as many people?

To some extent, experts say, Europeans are paying a price for living in open, affluent democracies, where people are used to free movement, easy travel and independent decision-making, and where governments worry about public opinion. Governments aren’t used to giving harsh orders, and citizens aren’t used to following them.

But China acted with a severity and breadth that stunned the West, making unpopular moves and accepting deep economic damage as the price of containing the disease. It closed off tens of millions of people, prohibiting them from leaving their cities and even their homes, except to get food and medical care, and it imposed lesser restrictions on hundreds of millions, shutting down whole industries in the process.

The Trump administration is asking state labor officials to delay releasing the precise number of unemployment claims they are fielding, an indication of how uneasy policymakers are about further roiling a stock market already plunging in response to the coronavirus outbreak.

In an email sent Wednesday, the Labor Department instructed state officials to only “provide information using generalities to describe claims levels (very high, large increase)” until the department releases the total number of national claims next Thursday.

The email, which was shared with The New York Times, noted that the reports were monitored closely by financial markets and should therefore remain embargoed. “States should not provide numeric values to the public,” wrote Gay Gilbert, the administrator of the department’s Office of Employment Insurance.

Ms. Gilbert has worked at the Labor Department under presidents of both parties, and there has been no indication that she was urged by political appointees to make the request. But President Trump has privately expressed irritation at the dire predictions of some of his advisers, most notably when Treasury Secretary Steven Mnuchin told lawmakers that unemployment could reach 20 percent this year.

Some states that received the guidance from Ms. Gilbert found it disturbing. It prompted at least one governor’s office, which shared the message on the condition of anonymity, to seek an opinion from the state attorney general about whether the state had to temporarily withhold the information.

In another state, lawmakers got a preview of the staggering numbers that are being withheld for the moment. In a private conference call Thursday with elected officials and union leaders, a top Pennsylvania labor official was blunt about the depth of the economic crisis, according to someone on the call.

Robert O’Brien, the state’s deputy secretary of labor and industry, said the government had been overwhelmed by a flood of unemployment insurance claims — 180,000 in the last few days. He said that was far more than the state usually gets in a whole month.

The situation may be even more dire in Washington State, the first center of the contagion in the United States. State officials there would only say they are seeing an “even more dramatic increase this week” after unemployment claims soared 150 percent last week.

The federal numbers released Thursday morning were already alarming: 281,000 people nationwide applied for unemployment insurance last week, up from 211,000 the previous week. They were apparently only a grim preview of what is to come.

The White House and lawmakers scrambled on Thursday to flesh out details of a $1 trillion economic stabilization plan to help workers and businesses weather a potentially deep recession, negotiating over the size and scope of direct payments to millions of people and aid for companies facing devastation in the coronavirus pandemic.

Senate Republicans, racing to put their imprint on the crisis response, unveiled a package that would provide hundreds of billions of dollars in loans to big corporations and small businesses, large corporate tax cuts and checks of up to $1,200 for taxpayers. The plan would also place limits on a paid-leave program enacted this week to respond to the crisis.

But the 247-page measure, the product of a feverish round of negotiations among Republicans, was all but certain to face opposition from Democrats who have pressed for more generous paid-leave benefits and targeting help to workers and families rather than large corporations.

The details emerged as Washington grappled with the dimensions of an extraordinary government rescue effort that is likely to last for many months. At the White House, President Trump said he would be open to having the government take equity stakes in companies that require federal help, a move that would be unpopular with shareholders and would give the government more oversight over businesses.

The State Department announced on Thursday that Americans should not travel outside the country, and that citizens abroad should either return home or stay in place as the coronavirus pandemic grows.

Some Americans overseas have urged the Trump administration to evacuate them.

The department raised its global health advisory to Level 4, or “do not travel,” which is the highest warning, usually reserved for nations that are war zones or facing serious disruptions such as political unrest or natural disasters.

The announcement is a recommendation, not a requirement. Millions of Americans are still overseas, and many are likely to opt to remain in place.

Some tourists or American citizens without long-term living arrangements or support networks abroad have been trying to get back to the United States, but have found that difficult because of border closings or flight cancellations and other transportation shutdowns. For example, American students trapped in Peru because of new travel restrictions imposed by the government there have been pleading with American officials to get them back to the United States.

President Trump on Thursday exaggerated the potential of drugs available to treat the new coronavirus, including an experimental antiviral treatment and decades-old malaria remedies that hint of promise but so far show limited evidence of healing the sick.

No drug has been approved to treat the new coronavirus, and doctors around the world have been desperately administering an array of medicines in search of something to help patients, especially those who are severely ill.

The malaria drugs, chloroquine and hydroxychloroquine, are among the remedies that have been tried in several countries as the virus has spread around the world, killing at least 9,800.

Doctors in China, South Korea and France have reported that the treatments seem to help. But those efforts have not involved the kind of large, carefully controlled studies that would provide the global medical community proof that these drugs work on a significant scale.

In a White House briefing Thursday, Mr. Trump said the anti-malaria drugs had shown “tremendous promise.”

The drugs’ potential has been highlighted during broadcasts on one of Mr. Trump’s favorite news channels, Fox News, where hosts like Laura Ingraham, Tucker Carlson and Jeanine Pirro have trumpeted the possibility of a real treatment.

But the F.D.A. has not approved any drugs for use in the treatment of coronavirus.

At a briefing on Thursday, Dr. Stephen M. Hahn, who has been the commissioner of the Food and Drug Administration for only three months, tended to walk back some of the president’s more inflated predictions that these drugs might vanquish the virus altogether.

He said Mr. Trump had asked the agency to look into chloroquine to fight the coronavirus, and that it was setting up a large clinical trial to evaluate the drug.

The peace agreement in Afghanistan, already stalled by delays in the release of Taliban prisoners and by a political stalemate in Kabul, is increasingly being defined and constricted by the coronavirus crisis.

The U.S. peace envoy, Zalmay Khalilzad, fired off a series of Twitter posts this week that cited the coronavirus as both an impediment to negotiations and a reason to urgently resolve political differences.

“It’s time for Afghans to compromise,” one of Mr. Khalilzad’s posts read. In another, he wrote, “coronavirus makes prisoner releases urgent.”

Mr. Khalilzad also acknowledged that disruptions triggered by coronavirus measures have made face-to-face negotiations increasingly difficult.

“Coronavirus and the resulting travel restrictions likely requires virtual engagement now,” he wrote.

Already, President Ashraf Ghani, 70, is spending time in self-imposed isolation in his private residence at the palace complex in Kabul. The palace Facebook site posted photos of Mr. Ghani alone at home, conferring with cabinet ministers and aides by video link.

Reaching out to provide assistance or charity in this trying time can ease your own anxiety too. Consider supporting local businesses, safely donating blood or reaching out in more creative ways.

Research and reporting was contributed by Edward Wong, David E. Sanger, David D. Kirkpatrick, Sui-Lee Wee, Katrin Bennhold, Richard Pérez-Peña, Tim Arango, Jill Cowan, Emily Cochrane, Jim Tankersley, Alan Rappeport, Maya Salam and David Zucchino.