“You have a virus that was able to establish itself in a dense social and sexual network and transmit efficiently because there’s no immunity,” said Anne Rimoin, professor of epidemiology at the University of California, Los Angeles.

A gradual decline of herd immunity against the closely related smallpox virus gave monkeypox more possibilities to jump from its natural animal hosts, infection-disease experts say. And one day, years ago, it infected someone who was part of a network with close physical contact between members—maybe a gay man with multiple sexual partners, or a sex worker—allowing it to spread sustainably among humans for the first time, these experts theorize.

That spread likely continued for years, undetected, until someone—or some people—with the virus traveled to large international events in Europe in May. Some attendees caught the virus and brought it back to their home countries, setting in motion the global outbreak that has now infected more than 29,000 people.