In a 2015 speech, Bill Gates warned that the greatest risk to humanity was not nuclear war but an infectious virus that could threaten the lives of millions of people.

That speech has resurfaced in recent weeks with 25 million new views on YouTube — but not in the way that Mr. Gates probably intended. Anti-vaccinators, members of the conspiracy group QAnon and right-wing pundits have instead seized on the video as evidence that one of the world’s richest men planned to use a pandemic to wrest control of the global health system.

Mr. Gates, 64, the Microsoft co-founder turned philanthropist, has now become the star of an explosion of conspiracy theories about the coronavirus outbreak. In posts on YouTube, Facebook and Twitter, he is being falsely portrayed as the creator of Covid-19, as a profiteer from a virus vaccine, and as part of a dastardly plot to use the illness to cull or surveil the global population.