But Mr. Trump did not alter his routine.

On Thursday, he met with the prime minister of Ireland, Leo Varadkar. At a news conference on Friday, he stood before a clutch of infection-control experts and business leaders in the White House Rose Garden.

The president shook hands with almost all of the business executives as he invited each to a shared microphone. Neither the handshakes nor the mic seemed particularly sanitary. One medical company executive was prepared — and offered his elbow even before Mr. Trump extended his hand.

The president responded with an elbow bump, saying, “I like that, that’s good.” Mr. Trump said Saturday that he was trying to break the habit of shaking hands, but that it was deeply ingrained.

“It’s a natural reflex,” he said, adding that he used to be a “non-hand-shaker,” but that “people walk up to me and want to shake my hand.”

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People who were exposed to a known coronavirus case should be asked to stay home and to monitor their health for 14 days, and be tested if they develop symptoms, according to the Infectious Diseases Society of America.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and a member of the White House Coronavirus Task Force, went further in a televised interview on Friday.

Any individual who has been exposed to someone infected with the coronavirus should be tested, he said. If testing is not possible, “you should try as best as you can to self-isolate,” Dr. Fauci said.