Mr. Gates, 64, the Microsoft co-founder, 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 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 Bill & Melinda Gates Foundation has pledged millions of dollars to companies developing potential novel coronavirus vaccines. In an interview on April 9 on CNBC, Gates spoke theoretically about how many people might experience side effects from...