For the next 15 days, Americans are urged to limit social gatherings, to avoid unnecessary travel and to stay indoors as much as possible, according to federal guidelines announced Monday to combat the spread of the coronavirus.

During a Monday afternoon press conference, President Donald Trump urged all Americans, including the "young and healthy," to avoid unnecessary exposure by staying indoors.

Americans are urged to limit social gatherings to no more than 10 people, to continue their education at home, to avoid discretionary travel and to avoid bars, restaurants and food courts, according to the guidelines from the U.S. Centers for Disease Control and Prevention.

The updated CDC guidelines will last for 15 days but the threat of the coronavirus spread in the United States could last until possibly August, according to Trump. Trump also added that a vaccine has begun phase one clinical trial.

U.S. State Department health official Dr. Deborah L. Birx said the millennial population is the "core" group in the United States that can help protect the nation's vulnerable population.

Older Americans who have had diseases, such as kidney, heart or lung diseases, and who have had immunological issues are most vulnerable, according to Birx, who added that a specific numerical age isn't the only appropriate method to determine who is most vulnerable.

When asked how he could rate his response to the coronavirus pandemic, Trump rated himself a 10 out of 10. Trump later said that the federal guidelines are "recommendations," not "orders," when asked whether the administration's response should be more strict.

Evan Vucci/AP Photo

On the economy, Trump said the United States would bounce back from the crisis but also acknowledged the possibility that the country could enter a recession.

The president also is pledging federal support for airlines struggling because of the pandemic, saying he’s “going to back the airlines 100%.”

Trump also says governors that need ventilators, respirators, masks and other equipment for medical professionals should first try to acquire the items on their own before turning to the federal government for help.

Officials across the country curtailed many elements of American life to fight the coronavirus outbreak on Sunday, with health officials recommending that groups of 50 or more don't get together and a government expert saying a 14-day national shutdown may be needed.

The governor of the only U.S. state without a confirmed case of the new coronavirus has declared a state of emergency so certain rules on personnel and purchasing could be waived.

West Virginia Gov. Jim Justice said Monday that he doesn't want the state to be "asleep at the switch” because it hasn't had a positive test.

About 20% of West Virginia’s population is over age 65, one of the highest rates in the nation.

Governors were closing restaurants, bars, and schools as the nation sank deeper into chaos over the crisis. Travelers returning home from overseas trips were stuck in line for hours at major airports for screenings, causing them to be crammed into just the kind of crowded spaces that public health officials have been urging people to avoid.

In a sign of the impending economic gloom on the horizon, the Federal Reserve slashed its benchmark interest rate to near zero. Trump sought to calm a jittery nation by declaring that the government has “tremendous control” over the situation and urging people to stop the panic buying of grocery staples that has depleted the shelves of stores around the country. Gun stores started seeing a similar run on weapons and ammunition as the panic intensified.

The U.S. surgeon general, meanwhile, said the number of coronavirus cases in the U.S. has reached the level that disease-battered Italy recorded two weeks ago — a signal that infections are expected to rise in America.

"We are at a critical inflection point in this country, people," U.S. Surgeon General Dr. Jerome Adams told Fox News. "When you look at the projections, there's every chance that we could be Italy."

As Americans struggled to come to terms with how to change their daily habits, the Centers for Disease Control and Prevention issued a dramatic recommendation: Because large events can fuel the spread of the disease, it said gatherings of 50 people or more should be canceled or postponed throughout the country for the next eight weeks. It added that, at any event, proper precautions should be taken, including making sure people are washing their hands and not getting too close.

But in a sign of the difficulty of striking the right balance, the statement from the CDC also said the recommendation does not apply to “the day to day operation of organizations such as schools, institutes of higher learning, or businesses.”

Even before the warning, parts of the country already look like a ghost town, and others are about to follow as theme parks closed, Florida beaches shooed away spring breakers, Starbucks said it will accept only drive-thru and takeout orders and the governors of Ohio, Illinois, New York, New Jersey, Connecticut, Washington and Massachusetts ordered bars to close and restaurants to stop dine-in service. California's governor asked the state's bars and restaurants to do the same, but didn't order it.

"The time for persuasion and public appeals is over,” Illinois Gov. J.B. Pritzker said. “This is not a joke. No one is immune to this.”

His decision came hours after Dr. Anthony Fauci, the federal government's top infectious disease expert, said he would like to see a 14-day national shutdown imposed to prevent the virus's spread.

“I think Americans should be prepared that they are going to have to hunker down significantly more than we as a country are doing,” said Fauci, a member of the White House task force on combating the spread of coronavirus. He heads the National Institute of Allergy and Infectious Diseases at the National Institutes of Health.

There is no indication Trump is considering such a move.

The worldwide outbreak has sickened more than 162,000 people and left more than 6,000 dead, with thousands of new cases confirmed each day. The death toll in the United States climbed to 64, while infections passed 3,200.

Meanwhile, harsh criticism rained on Trump and his administration Sunday from state and local officials over long lines of returning international passengers at some U.S. airports that could have turned them into coronavirus carriers as they tried to get home.

Chicago Mayor Lori Lightfoot lambasted the administration for allowing about 3,000 Americans returning from Europe to be stuck for hours inside the customs area at O'Hare International Airport on Saturday, violating federal recommendations from the Centers for Disease Control and Prevention that people practice “social distancing."

The passengers, many of them rushing home because of fears they would be stuck in Europe, were screened for coronavirus symptoms before they were allowed to leave the airport.

Long lines also formed Saturday in Boston, Dallas and others of the 13 airports that are accepting return flights from Europe.

“People were forced into conditions that are against CDC guidance and are totally unacceptable,” Lightfoot said.

Elizabeth Pulvermacher, a University of Wisconsin student, arrived Saturday at O'Hare from Madrid, where she had been studying and spent hours in line. The customs process made her feel “unsafe," she said.

“The whole idea is getting rid of the spread of coronavirus, but there were hundreds and hundreds of people in very close proximity,” Pulvermacher said.

Dr. Robert Murphy, executive director of Institute for Global Health at Northwestern University, said he was “appalled” by what he saw Saturday at nearby O'Hare.

“If they weren’t exposed to COVID-19 before, they probably are now. From a public health perspective, this is malpractice,” Murphy said in a statement Sunday. “The lack of preparation and concern is unfathomable.”

But the situation improved markedly Sunday at O'Hare and elsewhere. Acting Homeland Security Secretary Chad Wolf said wait times are down to 30 minutes after processes have been adjusted to better handle the influx and extra personnel sent to the airports that are “funneling” passengers returning from overseas.

Bailey Miles, a 20-year-old sophomore at Taylor University in Indiana, arrived also from Madrid fearing the worst, but she got through customs and a health screening in about an hour. She said officials seemed to have learned their lesson from Saturday, when some of her friends returned.

"The employees were really helpful, had positive attitudes and had a lot of grace," she said. She said a woman even passed out snacks.

At Newark Liberty International Airport in New Jersey, Amanda Kay said she was asked to keep her distance from other passengers when she arrived from Paris at Newark Liberty International Airport in New Jersey.

“They wanted 50 people at a time. So the first 50 people got off, and they asked us to keep a large distance between ourselves,” she said. “We showed them the first form and then as we walk there’s a person who has the scan for your temperature. And then you go through Customs and then that was it.”

Going forward, Wolf said he could not rule out a future halt to air travel within the U.S. Fauci said earlier Sunday on television that halting domestic travel had been discussed, though not seriously.

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.

And that may have given some people false hope, causing them to venture into crowds that Fauci and others would prefer they avoid. Even if someone doesn't become visibly ill, they can still carry the disease and spread it to others.

In New Orleans and Chicago, people clad in green for St. Patrick's Day packed bars and spilled onto crowded sidewalks on Saturday even after the cities canceled their parades.

New York City, which has the nation's largest public school district, announced that it will be closed starting Monday, joining most of the rest of the country. Mayor Bill de Blasio had originally balked, but under pressure from Gov. Andrew Cuomo and others he said Sunday, "I became convinced over the course of today that there is no other choice.”

In Florida, Walt Disney World and Universal-Orlando were closing Sunday night for the rest of the month, joining their California siblings, which already closed. Farther south, Miami Beach and Fort Lauderdale officials announced that they are closing their beaches, where thousands of college spring breakers from around the world have flocked. The cities also ordered restaurants and bars closed by 10 p.m. and to keep crowds below 250.

“We cannot become a petri dish for a very dangerous virus,” Miami Beach Mayor Dan Gelber said. “Spring break is over. The party is over.”

Starbucks said Sunday it is closing seating in its cafes and patio areas nationwide, but customers can still order at the counter, at drive-thrus or on the Starbucks app. It will also close or reduce hours in areas where there are a high-number of cases.

But not all government officials were as concerned. Oklahoma's Republican Gov. Kevin Stitt tweeted a picture of himself and his children at a crowded metro restaurant Saturday night.

Republican Rep. Devin Nunes had a similar message on Fox News Sunday, encouraging people to go to local restaurants and pubs despite the warnings of health officials.

Spencer reported from Fort Lauderdale, Florida. Associated Press journalists Mallika Sen, Vanessa Alvarez, Seth Wenig, Lisa Marie Pane, Christopher Weber, Eugene Johnson and John Seewer contributed.