The special packages arrived in January at the Bangkok laboratory of a virus expert named Supaporn Wacharapluesadee. They contained tubes of saliva and mucus from five people who had just landed at the city’s main airport from Wuhan, China. Days earlier, Chinese authorities had announced a cluster of mysterious pneumonia cases in Wuhan. Officials in Thailand, a top destination for Chinese tourists, rushed nurses to airports to screen arriving passengers for fevers or coughs. Health officials feared the culprit might be something nobody had ever seen.

“They asked me, could I detect the unknown or not?” says Wacharapluesadee.

There are as many as 1.6 million viruses we know nothing about lurking in mammals and birds, and as many as half might have