Dr. Anthony Fauci tests positive for the coronavirus. – The New York Times

Dr. Anthony S. Fauci, President Biden’s top medical adviser for the coronavirus pandemic, has tested positive for the virus and is experiencing “mild symptoms,” the National Institute of Allergy and Infectious Diseases said on Wednesday.

Dr. Fauci, the institute’s director, was positive on a rapid antigen test, the agency said in a statement. It added that he was fully vaccinated against the virus and had been boosted twice. He is taking Paxlovid, the Pfizer antiviral therapy authorized by the Food and Drug Administration for treatment of Covid-19, an agency spokeswoman said.

News that Dr. Fauci, one of the world’s foremost infectious disease specialists and a household name thanks to the pandemic, had fallen victim to the coronavirus reverberated across Washington and the country. The positive test was the first for Dr. Fauci, who is 81.

But with the Centers for Disease Control and Prevention estimating that more than half of Americans have contracted Covid-19, he is hardly the only big-name sufferer. Xavier Becerra, the secretary of health and human services, tested positive on Monday for the second time in less than a month. Representative Maxine Waters, Democrat of California, who is 83, announced on Tuesday that she had tested positive; she had also done so in April.

Dr. Fauci has not been in close contact with Mr. Biden or other senior government officials recently and will “isolate and continue to work from his home,” the statement from his institute said. He will return to his office once he tests negative.

But he had been making public appearances. The AIDS Clinical Trials Group — a network of hundreds of researchers conducting studies to improve treatment of H.I.V. and related infections — is meeting in Washington this week, and Dr. Fauci, whose laboratory work has been focused on H.I.V./AIDS, addressed the group in person on Tuesday.

Along with other top federal health officials, Dr. Fauci was expected to testify on Thursday before the Senate health committee on the state of the pandemic. An official said that Dr. Fauci’s institute was working with committee staff members to arrange for a remote appearance.