Bill Gates tests positive for the coronavirus.

Bill Gates, who has donated millions to pandemic relief efforts, tested positive for the coronavirus, he said on Tuesday.

Mr. Gates, 66, said on Twitter that he was isolating with mild symptoms and that he was vaccinated and boosted. It was not clear if this was the first time he had tested positive for the coronavirus. He also said that he would participate virtually in a meeting on Tuesday with people from his foundation.

In recent months, Mr. Gates, a co-founder of Microsoft and one of the richest men in the world, has focused his considerable resources on the pandemic. He published a book last week called “How to Prevent the Next Pandemic,” in which he explains how countries could pull off a coordinated effort to avert pandemics and “eliminate all respiratory diseases.”

He has been outspoken about how the global health authorities should respond to the pandemic and distribute vaccines. The Bill & Melinda Gates Foundation said it has donated millions to organizations including Gavi and the World Health Organization to fund testing, treatments and vaccine distribution. (The Gateses divorced last year, but both expressed continued commitment to the foundation.)

In January, Mr. Gates said on Twitter that “once Omicron goes through a country,” the virus could be treated “more like seasonal flu.”