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.

The wild claims have gained traction with conservative pundits like Laura Ingraham and anti-vaccinators such as Robert F. Kennedy Jr. as Mr. Gates has emerged as a vocal counterweight to President Trump on the coronavirus. For weeks, Mr. Gates has appeared on TV, on op-ed pages and in Reddit forums calling for stay-at-home policies, expanded testing and vaccine development. And without naming Mr. Trump, he has criticized the president's policies, including this week's move to cut funding to the World Health Organization.