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 at 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 arranging for employees to work from home.

“We are asking the American public to prepare for the expectation that this might be bad,” 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 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 Azar, the secretary of health and humans services, told a Senate panel on Tuesday. “And we need to be realistic about that.”

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 CDC 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, Messonnier said.

Addressing rising concerns, President Donald Trump on Tuesday said 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 CDC is forecasting.

Sen. John Kennedy, R-La., grew exasperated when the acting secretary of the Department of Homeland Security, Chad 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,” 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.

Azar said that there are 30 million N95 masks in the nation’s emergency stockpile, which typically cost less than $1 apiece.

Sen. Patty Murray, D-Wash., 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.”

China reported 508 new cases and another 71 deaths. The updates bring mainland China’s totals to 77,658 cases and 2,663 deaths.

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. On Tuesday, Iran said 15 people had died so far amid 95 confirmed cases.

But there are persistent doubts about the accuracy of infection figures reported by the Chinese and Iranian governments, raising the possibility that the true magnitude of the outbreak remains underreported.

By Tuesday, South Korea had reported 977 cases, the second most in the world, and the CDC warned Americans not to travel there. Most are in Daegu, the city at the center of the country’s outbreak.

In Italy’s north, where more than 200 were sickened, a dozen towns were sealed off and police wearing face masks patrolled.

Associated Press contributed.