The president acknowledged that the crisis — which has already killed thousands around the world and set off a plunge of world markets — could last until July or August and even plunge the nation into a recession. No country, including the United States, has it under control, he said, though he also suggested America could limit its death toll “if we do a really good job” responding now.

“Each and every one of us has a critical role to play in stopping the spread and transmission of the virus,” Trump said. “If everyone makes this change or these critical changes and sacrifices now, we will rally together as one nation, and we will defeat the virus, and we’re going to have a big celebration altogether. With several weeks of focused action, we can turn the corner and turn it quickly.”

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said that while some people may perceive the guidelines as inconvenient or going too far, they reflect a deteriorating assessment of the containment efforts and should be taken seriously.

“When you’re dealing with an emerging infectious diseases outbreak, you are always behind where you think you are if you think that today reflects where you really are. That’s not word speak. It means if you think you’re here,” Fauci said, gesturing to illustrate his point, “you’re really here because you’re only getting the results. Therefore, it will always seem that the best way to address it would be to be doing something that looks like it might be an overreaction. It isn’t an overreaction.”

The White House gave the country a 15-day window to flatten the soaring curve of infection, but some disease modelers see a trajectory that could create a crisis, similar to Italy, that would start to overwhelm the U.S. health care system in about 10 days.

Fauci called the 15 days “a trial” period for the guidelines to be reconsidered. “It isn’t that these guidelines are now gonna be in effect until July,” he clarified. “What the president was saying is the trajectory of the outbreak may go until then.”

The new guidelines come on the heels of Centers for Disease Control and Prevention guidance Sunday night that recommended the cancellation and postponement of gatherings of 50 people or more for the next eight weeks. It also comes after the Office of Personnel Management declared federal offices in the Washington, D.C., area open but with maximum flexibility for telework.

The president’s remarks reflect the increased urgency of the moment. As recently as Sunday, Trump was telling Americans to “relax” and that the pandemic would pass. And on Monday, White House economic adviser Larry Kudlow predicted that the outbreak would only impact the economy in the short term — “weeks and months,” he said, not years.

But hours later, Trump said the situation was “bad,” and administration officials pointed to new predictive modeling showing a need for more stringent social distancing measures. The case for aggressive social distancing is clear: A coronavirus vaccine is at least a year away, and the acceleration of the outbreak within the U.S. has put the it on track with some of the world’s hardest-hit nations.

“If everybody does what we’ve asked for in the next 15 days, we will see a dramatic difference,” said Deborah Birx, the White House’s coronavirus response coordinator.

More than a month-and-a-half into the response effort and after failed bids to prevent and then contain the disease’s spread, the Trump administration is now scrambling to put its full weight behind a messaging campaign aimed at convincing people to do something simple yet unnatural: stay home.

Still, public health experts said the administration has much work to do to sell Americans on such an abrupt change in habits and rebuild its credibility after weeks of mixed messages.

Trump was joined at the briefing by his key health advisers, including Fauci, Birx and Vice President Mike Pence, who is leading the administration’s response.

A growing number of governors earlier Monday followed the lead of fellow executives, announcing closures of bars, restaurants and other venues in states across the country. The new guidance similarly calls for any bars, restaurants, food courts, gyms and other venues to be closed in areas where there’s evidence of community transmission.

Trump held separate video teleconferences with G-7 leaders and governors on Monday morning to discuss the response to the coronavirus.

“Went very well,” he tweeted after speaking to governors.

During the call, however, Trump urged governors to try to get respirators, ventilators and other equipment themselves. Hospital systems have raised alarms over a lack of beds and medical supplies to handle the crush of anticipated patients, and already some hospitals are beginning to postpone certain procedures in an effort to free up space and medical personnel.

“We will be backing you, but try getting it yourselves,” the president told them, according to a source in the room. “Point of sales, much better, much more direct if you can get it yourself.”

The New York Times first reported the president’s remarks to governors.

Trump told reporters that governors would be able to get what they need sooner if they don’t rely on the federal government. Gov. J.B. Pritzker of Illinois, a Democrat, said while “the words did not quite come out right,” the president encouraged states to use whatever supply chains they have access to while the federal government attempts to maintain more respirators and ventilators.

Administration officials also stressed the important role millennials can play in combating the spread of the virus. Birx called that generation “the core group that will stop the virus.”

She said millennials are the most likely group to be the least symptomatic but also the group most likely to be out and about and in social gatherings. That means they could unknowingly spread the virus to older people or those with underlying issues, who are more vulnerable, if they ignore guidance and recommendations, but they’re also adept at communicating via text and across social media platforms.

“Public health people like myself don’t always come out with compelling and exciting messages that a 25- to 35-year-old may find interesting and something that they’ll take to heart,” Birx said. “But millennials can speak to one another about how important it is in this moment to protect all of the people.”

Surgeon General Jerome Adams told CNN on Monday morning that the U.S. was “at a critical inflection point,” noting that America now has the same number of cases that Italy had two weeks ago. Italy is now on lockdown as it deals with more than 27,000 cases of the coronavirus, trailing only China, where the outbreak began.

And public health experts have warned that without further dramatic steps, the U.S. could look like Italy by month’s end, with hospitals swamped by sick patients and hundreds of thousands of infected people freely circulating.

“We have a choice to make: Do we want to really lean into social distancing and mitigation strategies and flatten the curve or do we just want to keep going on with business as usual and end up being Italy?” Adams said.

As of Monday afternoon, more than 4,100 cases have been confirmed in the U.S., though experts expect that number to rise as more testing becomes available. More than 70 people have died.

Rachel Roubein and Dan Goldberg contributed to this report.