WASHINGTON — When an outbreak of the Ebola virus touched the United States’ shores in mid-2014, Donald J. Trump was still a private citizen. But he had strong opinions about how America should act.

Mr. Trump, who has spoken openly about his phobia of germs, closely followed the epidemic, and offered angry commentary about what he said was the Obama administration’s dangerous response. He demanded draconian measures like canceling flights, forcing quarantines and even denying the return of American medical workers who had contracted the disease in Africa.

“Ebola patient will be brought to the U.S. in a few days — now I know for sure that our leaders are incompetent. KEEP THEM OUT OF HERE!” Mr. Trump tweeted on that July 31 after learning that one American medical worker would be evacuated to Atlanta from Liberia. “The U.S. cannot allow EBOLA infected people back,” Mr. Trump wrote the next day, adding: “People that go to far away places to help out are great — but must suffer the consequences!”

In nearly 50 tweets, as well as in appearances on Fox News and other networks, Mr. Trump supported flight bans and strict quarantines and branded President Barack Obama’s deployment of troops to West Africa to fight the disease as “morally unfair.”

Many health experts called Mr. Trump’s responses extreme, noting that the health workers would have most likely faced agonizing deaths had they not been evacuated to American hospitals. Former Obama administration officials said his commentary stoked alarmism in the news media and spread fear among the public.

Now Mr. Trump confronts another epidemic in the form of the coronavirus, this time at the head of the country’s health care and national security agencies. The illness has infected few people in the United States, but health officials fear it could soon spread more widely. And while Mr. Trump has so far kept his distance from the issue, public health experts worry that his extreme fear of germs, disdain for scientific and bureaucratic expertise and suspicion of foreigners could be a dangerous mix, should he wind up overseeing a severe outbreak at home.

“Having a head of state who is trusted, who is a credible message deliverer, consistent in communications and consistent with evidence, is absolutely necessary,” said Dr. Jennifer Nuzzo, a senior scholar at the Johns Hopkins Center for Health Security. “There’s so much misinformation out there, so a central role is for a leader to be a go-to source for credible information.”

For the most part, Mr. Trump has been uncharacteristically restrained in his commentary about the virus, delegating the response to senior health officials.

At the end of January, Mr. Trump created a 12-member coronavirus task force, which will be managed by the National Security Council. It includes the health and human services secretary, Alex M. Azar II; Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health; and Dr. Robert R. Redfield, the director of the Centers for Disease Control and Prevention.

All three have experience dealing with infectious diseases, especially Mr. Fauci, who has helped to manage the response to numerous outbreaks, including the AIDS epidemic, the SARS virus and Ebola.

In many of his remarks he has made, Mr. Trump has praised President Xi Jinping of China, even though his government has been widely criticized for a clumsy and initially secretive response to the coronavirus, and made some questionable announcements.

“They’re working really hard, and I think they’re doing a very professional job,” Mr. Trump said on Friday.

Speaking to a meeting of the nation’s governors on Monday, he predicted that the virus will have run its course by spring and again referred to the Chinese president.

“The virus that we’re talking about having to do, a lot of people think that goes away in April, with the heat, as the heat comes in, typically that will go away in April,” Mr. Trump said. Referring to the United States, he added: “We’re in great shape, though. We have 12 cases, 11 cases, and many of them are in good shape now.” (The number of confirmed cases is 12, according to a Monday update by the Centers for Disease Control and Prevention.)

“I had a long talk with President Xi two nights ago,” he added. “He feels very confident. He feels that again, as I mentioned, by April or during the month of April, the heat generally speaking kills this kind of virus. So that would be a good thing.”

Public health experts questioned the speculative nature of his comments. “I think there is a lot we still don’t know about this virus, and I’m not sure we can say definitively that it will dissipate with warmer weather,” said Dr. Rebecca Katz, director of the Center for Global Health Science and Security at Georgetown University.

“Relying on the fact that it’s going to warm up in April as reassurance that the virus will be controlled by then I think is arguable,” added Dr. James M. Hughes, a professor emeritus of medicine at Emory University.

Other comments from Mr. Trump about the disease have been inaccurate or met with criticism. In late January, he wrote on Twitter that five coronavirus cases had been confirmed in the United States just hours after a sixth had been confirmed. Addressing the virus on Feb. 2, Mr. Trump boasted to Fox News, “We pretty much shut it down coming in from China.”

The words “shut it down” apparently referred to an executive order the president had issued two days earlier, barring entry to the United States by foreign citizens who traveled to China in the past two weeks. Some health experts worry that Mr. Trump overpromised because the order — which the White House announced abruptly, with little outside consultation — is unlikely to prevent the illness from reaching the United States, and federal health officials say they assume the number of cases in the United States is likely to increase.

Speaking at a Friday news briefing, Mr. Azar defended the president’s actions and said the new travel restrictions were “very measured and incremental” while praising Mr. Trump’s “aggressive” response overall.

Presidential words have played an important role in past health crises. Ronald Reagan was severely criticized for his slow response to the spread of H.I.V. and for recommending abstinence to address the infection. Mr. Obama resisted pressure from Mr. Trump and others to institute sweeping travel bans and quarantines, calling them alarmist and urging levelheaded thinking.

“This is a serious disease, but we can’t give in to hysteria or fear because that only makes it harder to get people the accurate information they need,” Mr. Obama said in October 2014 in a radio address. “We have to be guided by the science. We have to remember the basic facts.”

Mr. Trump’s record on presenting facts has been a persistent source of criticism in the scientific community. The last time the White House became involved in managing a national emergency, during Hurricane Dorian in September, he misstated an official forecast of the storm’s path, then displayed a tracking map in the Oval Office that he appeared to have altered with one of his signature Sharpie pens.

“Trump has the right people, but the wrong instincts and the wrong structure,” said Ronald Klain, who directed the Obama administration’s response to the 2014 Ebola crisis. “Our government is staffed with the best experts, scientists and medical leaders in the world. But Trump’s instincts — anti-science, anti-expert, isolationist and xenophobic — risk that he will eschew that advice at critical points.”

Another factor is Mr. Trump’s lifelong obsession with personal hygiene. While he has shown little interest in health or science policy, he has often spoken of his extreme revulsion to germs.

In his 2004 book, “How to Get Rich,” the president declared himself “very much of a germophobe,” and wrote that he was “waging a personal crusade to replace the mandatory and unsanitary handshake with the Japanese custom of bowing.”

As a result, Mr. Trump generally avoids the political tradition of shaking dozens of hands after his speeches and rallies, and frequently uses hand sanitizer. He is quick to banish aides who cough and sneeze in his presence. In a January 2017 interview, the president’s personal physician, Dr. Harold N. Bornstein, said Mr. Trump always “changes the paper himself” in the examining room.

In that regard, Mr. Trump may have gained at least a temporary ally in Mr. Xi. Mingling for the cameras with Beijing residents Monday, Mr. Xi, who sported a surgical mask, recommended they skip the customary form of greeting. “Let’s not shake hands in this special time,” he said.

Lawrence K. Altman contributed reporting.

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