“We don’t understand why some young healthy people progress to severe disease and even die, and others don’t,” he says. “Never, never underestimate a new disease, there’s just too much unknown.”

Aylward’s warning comes on the heels of the release of a preliminary analysis on Wednesday by the Centers for Disease Control and Prevention (CDC) that suggests 38% of coronavirus patients in the U.S. who were sick enough to be hospitalized were under the age of 55.

“Ten percent of the people who are in ICUs in Italy are in their 20s, 30s or 40s, these are young, healthy people with no co-morbidities, no other diseases,” he says.

The doctor, who lead a joint WHO-China mission into China in February to study the effectiveness of the coronavirus response in the country, says frontline doctors there couldn’t pinpoint what factors were leading the virus to kill some young people. “They said, ‘We simply don’t know.’”