The idea that sars-cov-2, the virus that causes covid-19, instead leaked from a Chinese research laboratory has persisted since the earliest days of the pandemic. Though this theory has rested largely on rumours and circumstantial evidence, it has nevertheless set the stage for a bitter three-and-a-half year row. New diseases in people almost always come from close contact between human populations and animals. A “spillover” happens when viruses from an animal species jump into humans and then go on to wreak havoc. Where did covid-19 come from?