As COVID-19 sweeps the world, related viruses quietly circulate among wild animals. A new study shows how SARS-CoV-2, the virus that causes COVID-19, and SARS-CoV-1, which caused the 2003 SARS outbreak, are related to each other. The work, published recently in the journal Virus Evolution, helps scientists better understand the evolution of these viruses, how they acquired the ability to infect humans and which other viruses may be poised for human spillover.