evidence: They couldn’t rule out an accidental leak of a natural virus from a lab that was studying it, but thanks in part to research from the Wuhan Institute of Virology (WIV), it was also known that coronaviruses that had the capacity to infect people, and that were related both to the one that caused the original severe acute respiratory syndrome (Sars) epidemic in 2003, and to Sars-CoV-2, were circulating in Chinese bat populations. [...] But since RaTG13 was identified, others have been isolated from bats in China and south-east Asia that are also highly similar to Sars-CoV-2. A cluster of them, from the Chinese province of Yunnan, was described last week in the journal Cell. "The connection between RaTG13, the Wuhan Institute of Virology and Sars-CoV-2 isn’t required any more," says Robertson. [...] This leaves the lab leak theory resting principally, for now, on unverified reports of three cases of respiratory illness among the WIV’s nearly 600 staff in November 2019, a winter month in Wuhan, and the fact that the institute took a database of viral genome sequences offline two months earlier – to protect it from hackers, they told WHO investigators.