It’s been more than ten years since the first cases of SARS (severe acute respiratory syndrome) were identified in southern China. It spread to four continents and infected at least 8,200 people before heroic efforts finally brought its progress to a halt. Since then, scientists have sequenced the genome of the virus behind the disease—a coronoavirus called SARS-CoV—and teased apart its infectious tricks. But one lingering question remained: Where did it come from?

At first, the answer seemed straightforward. When investigators swept Chinese animal markets, they found SARS-CoV in palm civets. The animals were quickly culled in their thousands, but later studies showed that their wild or farmed relatives don’t actually carry the virus. They may have acted