WASHINGTON >> Racing to stall an expected surge of coronavirus cases, the White House today released a series of sweeping guidelines that for the next 15 days will temporarily rewrite the norms of American society.

President Donald Trump, employing a newly somber tone about the crisis enveloping the globe, urged all older Americans to stay home and everyone to avoid crowds and eating out at restaurants. The president for the first time acknowledged that the pandemic may send the economy into a recession and suggested that the nation may be dealing with the virus until “July or August.”

The guidelines were released to an uneasy country. The stock market had its worst day since 1987, America’s largest school system shut its doors and questions remained about the administration’s ability to test for the virus and provide hospital space for those who fall ill.

Among the new recommendations: Over the next half month, Americans should not gather in groups of more than 10 people, schooling should be at home and discretionary travel and social visits should be avoided. If anyone in a household tests positive for the virus, everyone who lives there should stay home. The administration did not say how old people should be to follow the advice to stay home.

“We will rally together as one nation and we will defeat the virus,” Trump said. “We can turn the corner and turn it quickly.”

The president, when asked when the pandemic would subside, said that “if we do a really good job” the crisis could pass by the height of summer. Dr. Anthony Fauci, the administration’s infectious disease expert, did not challenge that claim.

Trump’s prediction was far less optimistic than his earlier ones, when he insisted it could be over in weeks. He also acknowledged that the virus may send the nation’s economy into a recession, a potentially brutal blow for an incumbent seeking reelection.

That admission was startling for Trump, who for two weeks had struck a discordantly optimistic tone about the pandemic that endangered his presidency. Today that upbeat demeanor was gone as he acknowledged the virus was “not under control” in the United States or globally.

He said he did not yet plan to call for domestic travel restrictions while his administration made pointed appeals to millennials to stop socializing and risk spreading the virus. He was reluctant to advise states to cancel the primary elections ahead. And without providing details, he said “we’re going to back the airlines 100%,” a note of reassurance for an industry crippled by travel bans and fears of spreading the virus.

The expansive guidelines were issued on a day of fast-moving developments in a capital resplendent in cherry blossoms but awash in anxiety.

Congress convened to try to finish an aid package and consider another one behind it. The Supreme Court postponed oral arguments for the first time in over a century. And the U.S. surgeon general said that the United States is about where Italy was two weeks ago in the coronavirus struggle, a sign that infections are expected to rise.

“When you look at the projections, there’s every chance that we could be Italy,” Surgeon General Dr. Jerome Adams told Fox News. Yet he said the U.S. has opportunities to mitigate the pandemic.

Two weeks ago, Italy had 1,700 cases of coronavirus and had reported 34 deaths. Now, Italy is reporting an estimated 25,000 cases and more than 1,800 deaths. About 3,800 cases have been reported in the United States and so far, more than 65 people have died from coronavirus. Although the numbers may not be directly comparable, the trajectory is, as Adams sees it.

The administration also alleged that a foreign disinformation campaign was underway aimed at spreading fear among Americans already spooked by the pandemic.

On Sunday, federal officials began confronting what they said was a deliberate effort by a foreign entity to make Americans think the government was going to impose a nationwide quarantine. Agencies took coordinated action Sunday evening to assure Americans that the government was not planning to do that.

And today, national security officials said there had been a cyber attack involving the computer networks of Health and Human Services, but the networks were operating normally. They didn’t detail the nature or scope of the problem or identify the foreign entity.

On Fox News, Adams claimed the U.S. has “turned the tide” on testing, a critical part of tracking and containing pandemics, but whether that is true remains to be seen. The U.S. effort has been hobbled by a series of missteps, including flaws with the testing kits first distributed by the federal government and bureaucratic hurdles that held up testing by private laboratories.

On Capitol Hill, a nearly empty House met to adopt a resolution to make what were described as technical changes to polish off a coronavirus response measure designed to speed testing for the virus and provide sick leave to workers kept home because of it. It went into recess before adopting the measure, however.

The measure is likely to pass the Senate this week, though the timing is uncertain. The 100 senators, with a few exceptions, were expected to gather this evening, despite the federal public-health guidelines prohibiting large gatherings.

Talks are already underway on another bill to try to blunt the damage the virus is doing to the economy. Senate Majority Leader Mitch McConnell said it will focus on direct financial help to individuals, help for businesses such as the airline industry, and further steps to support the health system.

Nearby, the Supreme Court said it is postponing arguments for late March and early April because of the coronavirus, including a round over subpoenas for Trump’s financial records. The last time oral arguments were postponed was in 1918, due to the Spanish flu epidemic. In 2001, they were relocated, but not postponed, after an anthrax scare.

Within the White House, the pandemic altered daily life.

Everyone who entered the complex was screened for a fever and reporters were spaced out in the briefing room to reduce the chance of spreading the virus. And the outgoing acting chief of staff, Mick Mulvaney, isolated himself in South Carolina as his niece, who had flu-like symptoms and lives with him in Washington, awaited test results to see whether she has the virus. Mulvaney has tested negative, but was isolating himself “out of an abundance of caution due to his proximity to the President,” White House press secretary Stephanie Grisham said.

Larry Kudlow, director of the National Economic Council, said the White House is working toward $800 billion in fiscal relief, with about half of that having been approved through executive actions and legislation and much of the remainder being sought through a payroll tax cut that has been met coolly by both parties in Congress.

The Federal Reserve said Sunday it was taking emergency action to slash its benchmark interest rate to near zero. But the Fed’s move did not calm markets. The Dow Jones Industrial Average finished today down nearly 3,000 points.

For most people, the new coronavirus causes only mild or moderate symptoms, such as fever and cough. For some, especially older adults and people with existing health problems, it can cause more severe illness, including pneumonia.

The vast majority of people recover. According to the World Health Organization, people with mild illness recover in about two weeks, while those with more severe illness may take three weeks to six weeks to recover.

The worldwide outbreak has sickened more than 180,000 people and killed more than 7,100.

Trump in recent days has imposed sweeping travel restrictions for much of Europe. On Saturday, he added Britain and Ireland to a list of countries facing travel restrictions over the next 30 days.