In an interview with The Associated Press, Dr. David Heymann, who formerly headed WHO’s emergencies department, said the leading theory to explain the spread of the disease was sexual transmission among gay and bisexual men at two raves held in Spain and Belgium. Monkeypox has not previously triggered widespread outbreaks beyond Africa, where it is endemic in animals.

“We know monkeypox can spread when there is close contact with the lesions of someone who is infected, and it looks like sexual contact has now amplified that transmission,” said Heymann.

That marks a significant departure from the disease’s typical pattern of spread in central and western Africa, where people are mainly infected by animals like wild rodents and primates and outbreaks have not spilled across borders.