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 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.

The news caps weeks of fear that the coronavirus spreading from China may become a pandemic, disrupting the global economy and political landscape in ways that are difficult to forecast.

Iran, South Korea and Italy are now grappling with clusters of infection, even as the epidemic in China’s Hubei province seems to be slowing. In the United States, stocks tumbled Monday as the dimensions of the outbreak became clearer.

Officials at the CDC said they did not know whether the 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, she added.

“We are asking the American public to prepare for the expectation that this might be bad,” Messonnier said.

In Washington, the secretary of health and human services told a Senate panel that federal and local health departments will need as many as 300 million masks for health care workers and additional ventilators for hospitals to prepare for an outbreak of coronavirus in the U.S.

“This is an unprecedented potentially severe health challenge globally,” Alex Azar, the health and human services secretary, told a Senate subcommittee.

But lawmakers from both parties made it clear they were unconvinced the Trump administration was prepared for the outbreak that the CDC is forecasting.

As Sen. John Kennedy, R-La., pressed for an exact number of people expected to be infected, the acting secretary of the Homeland Security Department, Chad Wolf, could not answer.

“I’m all for committees and task forces, but you’re the secretary,” Kennedy responded. “I think you ought to know that answer.”

Trump 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.

“We’re really learning day by day and week by week of the contours of this disease,” Azar said.

Lawmakers listening to Azar’s requests appeared startled.

Sen. Patty Murray 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.”

Azar said that officials currently had 30 million N95 masks in the nation’s emergency stockpile, which typically cost less than $1.

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.

“We cannot hermetically seal off the United States to a virus,” Azar said. “And we need to be realistic about that.”

Messonnier said that she had sat down with her children and told them, “we as a family need to preparing for significant disruption of our lives.”

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.”