The coronavirus almost certainly will begin spreading in communities in the United States, and Americans should begin preparations now, officials at the Centers for Disease Control and Prevention said on Tuesday.
“It’s not so much of a question of if this will happen anymore but rather more of a question of exactly when this will happen,” Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases, said in a news briefing.
In the event of an outbreak, communities should plan for “social distancing measures,” like dividing school classes into smaller groups of students, closing schools, canceling meetings and conferences, and arranging for employees to work from home.
“We are asking the American public to prepare for the expectation that this might be bad,” Dr. Messonnier said.
China’s battle to contain the epidemic has shown signs of success, with a plunge in the rate of new infections. But this positive trend was overshadowed by the sudden appearance of clusters of infections in Iran, South Korea and Italy, underlining the threat of a global pandemic racing out of control.
The emergence of these new hubs underscored the lack of a coordinated global strategy to combat the coronavirus, which has infected nearly 80,000 people in 37 countries, causing at least 2,600 deaths.
“We cannot hermetically seal off the United States to a virus,” Alex M. Azar II, the secretary of health and humans services, told a Senate panel on Tuesday. “And we need to be realistic about that.”
Stocks plunged for the second day in a row, down nearly 3 percent by Tuesday afternoon, a decline that put the S&P in the red for the year.
As recently as last Wednesday, the index was at a record high. But since then, growing outbreaks in Europe and elsewhere in Asia have raised fears that the virus will continue to be a drag on drag on the global economy.
At the moment, the United States has just 57 cases, 40 of them connected to the Diamond Princess, the cruise ship overwhelmed by the coronavirus after it docked in Japan. Those patients are in isolation in hospitals, and there are no signs of sustained transmission in American communities yet.
Officials at the C.D.C. said they did not know whether spread of the disease to the United States would be mild or severe. But Americans should be ready for a significant disruption to their daily lives, Dr. Messonnier said.
President Trump, traveling overseas, had a sharply different tone than his own health officials, saying at a news briefing in India: “You may ask about the coronavirus, which is very well under control in our country.”
“We have very few people with it and the people that have it are, in all cases, I have not heard anything other — the people are getting better, they’re all getting better.”
But on Tuesday, members of the administration were grilled on Capitol Hill by lawmakers from both parties, who made it clear they were not convinced that the Trump administration was prepared for the outbreak that the C.D.C. is forecasting.
Senator John Kennedy, Republican of Louisiana, grew exasperated when the acting secretary of the Department of Homeland Security, Chad F. Wolf, could not say how many people were expected to become infected.
“I’m all for committees and task forces but you’re the secretary,” Mr. Kennedy responded. “I think you ought to know that answer.”
The administration officials overseeing the response to a potential coronavirus outbreak told lawmakers that the initial amount of money requested by the White House — $1.25 billion in new funds and $1.25 billion taken from other programs — would likely be just a first round.
Mr. Azar said that there are 30 million N95 masks in the nation’s emergency stockpile, which typically cost less than $1 apiece.
Senator Patty Murray, Democrat of Washington, asked the health secretary whether he thought the United States currently had enough health masks in stock.
“Of course not,” he responded, “or else we wouldn’t be asking for more.”
Mr. Azar said he was alarmed by the human-to-human transmission of the virus in other parts of the world without an identifiable connection to confirmed cases, and what that could mean for how the virus may hit the United States in the coming months.
Dr. Messonnier said that because there is currently no vaccine or treatment for the new coronavirus, communities and individuals should prepare other means of protecting themselves.
Dr. Messonnier said that she had sat down with her children and told them, “we as a family need to prepare for significant disruption of our lives.”
Individually, people can take the measures recommended for other infectious diseases, like washing their hands, covering their mouths when they cough, and staying home and away from others when they are sick.
Americans “should ask their schools about plans for dismissal” and for conducting classes online in the event that the spread of illness becomes serious, she said.
“I contacted my local school superintendent this morning with exactly those questions.”
The World Health Organization said that the pace of confirmed new cases in China, which exceeded 2,000 a day a month ago, had dropped steadily, to a low of 508 on Monday.
The severe measures imposed by the Chinese authorities to isolate patients and the hardest-hit areas had likely prevented hundreds of thousands of additional infections, the W.H.O. officials added.
But W.H.O. officials have also warned that the world is unprepared for a leap in infections, which could overwhelm medical resources in many countries. They also cautioned that new cases could suddenly resurge in China, as the government struggles to get people back to work.
And there are persistent doubts about the accuracy of infection figures reported by the Chinese government, raising the possibility that the true magnitude of the outbreak remains underreported.
By Tuesday, South Korea had reported a total of 893 cases, the second most in the world, and the C.D.C. warned Americans not to travel there.
Of the 60 new cases reported by South Korea’s federal health agency, 49 came from Daegu, the city at the center of the the country’s outbreak.
In Iran, a spike in coronavirus infections — including to the top health official in charge of fighting the disease — has prompted fears of a contagion throughout the Middle East. In Italy, one of Europe’s largest economies, officials are struggling to prevent the epidemic from paralyzing the commercial center of Milan.
Rick Gladstone contributed reporting from New York, and Zolan Kanno-Youngs and Emily Cochrane from Washington.