WASHINGTON — Dr. Anthony S. Fauci, the nation’s leading expert on infectious diseases, who has become a regular at President Trump’s coronavirus briefings, will receive enhanced personal security after receiving threats following his repeated pleas for Americans to help slow the spread of the deadly pandemic, officials said on Wednesday.

Dr. Fauci has been the Trump administration’s most outspoken advocate of social distancing rules that have shuttered the nation’s schools, forced businesses to close, kept people in their homes and battered the United States economy.

That has made him a target of online conspiracy theorists who have accused Dr. Fauci, a longtime scientist and civil servant who has served presidents of both parties, of trying to undermine Mr. Trump during a year in which the president is fighting for re-election.

The Department of Health and Human Services granted the enhanced personal security for Dr. Fauci, 79, after the Justice Department signed off on a request for extra agents to guard him, officials said. Alex M. Azar II, the health and human services secretary, had grown worried that the threats against the doctor were increasing as more of the country shut down in response to the coronavirus.

“Yesterday, upon the recommendation of the U.S. Marshals Service, the department approved the special deputization request from H.H.S. for nine H.H.S.-O.I.G. special agents to provide protective services for Dr. Fauci,” the Justice Department said in a statement, referring to the Office of the Inspector General at the health department. The Washington Post first reported the news of the enhanced security.

Caitlin Oakley, a spokeswoman for the health department, would not confirm Dr. Fauci’s enhanced security but called him “an integral part of the U.S. government’s response against Covid-19,” referring to the disease caused by the coronavirus.

“Among other efforts,” she added, “he is leading the development of a Covid-19 vaccine and he regularly appears at White House press briefings and media interviews.”

The coronavirus could kill 100,000 to more than 200,000 Americans in the coming weeks. Dr. Fauci, along with a fellow task force member, Dr. Deborah L. Birx, encouraged Mr. Trump to extend the amount of time that the country would remain shuttered through the end of this month. Some states, like Virginia, have issued stay-at-home orders that do not expire until further into the spring.

The idea that Dr. Fauci is responsible for the country’s hardship — and for whatever political damage Mr. Trump might suffer as a result — was fueled in part by a moment during a news briefing in which Dr. Fauci dropped his head and touched his forehead as the president was speaking. The image went viral online and right-wing supporters of Mr. Trump cited it as evidence that Dr. Fauci sought to undercut the president.

One anti-Fauci Twitter post last week said, “Sorry liberals but we don’t trust Dr. Anthony Fauci.”

The hashtag #FauciFraud has been used by more than 70 Twitter accounts, some posting hundreds of times a day, according to an analysis by The New York Times. Criticism of Dr. Fauci has also come from leading conservative voices and supporters of Mr. Trump, including Tom Fitton, the president of Judicial Watch, a conservative group; Bill Mitchell, the host of the far-right online talk show “YourVoice America”; and Shiva Ayyadurai, who has falsely claimed to be the inventor of email.

But Mr. Trump himself has praised the doctor. During Wednesday’s briefing, when a reporter asked about Dr. Fauci’s personal security, the president said: “He doesn’t need security. Everybody loves him.”

In fact, Dr. Fauci has earned plaudits from many medical experts and public health officials for his often grim assessment of the threats facing the United States from the coronavirus — even sometimes contradicting the president’s rosier outlook.

For weeks, Dr. Fauci’s remarks on television and at the White House stood in sharp contrast to the commentary from Mr. Trump’s fiercest supporters on Fox News, whose leading hosts repeatedly claimed that Democrats, the news media and public health experts were inflating the threat of the virus.

After the viral video of Dr. Fauci lowering his head, online attacks against him increased. A seven-year-old email that he wrote to an aide of former Secretary of State Hillary Clinton’s was posted online by The American Thinker, a conservative blog. In the email, Dr. Fauci praised Mrs. Clinton for her stamina during hearings into the 2012 attacks on the American compound in Benghazi, Libya. The blog suggested falsely that the email proved that Dr. Fauci was part of a secret group who opposed Mr. Trump.

Ironically, in the past several days, the president has largely adopted Dr. Fauci’s more dire warnings about the dangers of the rapidly spreading virus. On Tuesday, Mr. Trump called it a “great national trial unlike any we have ever faced before” and echoed Dr. Fauci’s language about the need to minimize its spread.

“It’s a matter of life and death, frankly,” Mr. Trump said, offering a sober assessment of the pandemic’s effect. “It’s a matter of life and death.”