LONDON (AP) A leading adviser to the World Health Organization described the unprecedented outbreak of monkeypox in developed countries as a random event that might be explained by sexual behavior at two recent raves in Europe.

Dr. David Heymann, who formerly headed WHOs emergencies department, told The Associated Press that the leading theory to explain the spread of the disease was sexual transmission at 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.