She said Mr. Trump’s attempts to minimize the threat posed by the coronavirus was a dramatic departure from the way most political figures have approached past health emergencies

“We’ve never had a political leader say stuff like this,” she said.

But, she added, “At the same time, what we can’t do is just have media messages that focus on his words and not address practical things that people can do,” concrete information that she said everyone, Trump supporters and critics alike, are hungry for.

That national outlets may be more alarmist and politicized than local ones is common to nearly every epidemic she has studied. But what sets this one apart from most of those is that it is unfolding on Facebook and Twitter as well.

“The most alarming messages have come from just people speculating on social media and other people taking that as fact,” she said.

Already, a number of Democratic voters said they had little confidence in Mr. Trump’s public statements.

“I don’t think he gives a damn,” said Shelli Hunt, 62, a saleswoman for a cable company in Las Vegas who voted for Bernie Sanders in last month’s Democratic caucuses. “It’s all about the spin. If he spent half of the energy he does running the country that he does into tweeting and blaming people, we’d be in a lot better shape.”

Other experts questioned whether Mr. Trump has the credibility to guide the country through a public-health crisis given his history of making false claims. After Hurricane Maria shattered Puerto Rico in 2017 leaving thousands homeless or without power for months, Mr. Trump hailed himself in 2019 as “the best thing that’s ever happened to Puerto Rico” and disputed the official estimates that about 3,000 people died because of the storm.