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 WHO’s emergencies department, told The Associated Press that the leading theory to explain the spread of the disease was sexual transmission among men 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.

This 2003 electron microscope image made available by the Centers for Disease Control and Prevention shows mature, oval-shaped monkeypox virions (left) and spherical immature virions (right) obtained from a sample of human skin associated with the 2003 prairie dog outbreak. A leading doctor who chairs a WHO expert group described the unprecedented outbreak of monkeypox in developed countries as “a random event” that might be explained by sexual behavior at two recent mass events in Europe. (Cynthia S. Goldsmith, Russell Regner/CDC via AP, File)

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.

Health officials say most of the known cases in Europe have been among men who have sex with men, but scientists say it will be difficult to disentangle whether the spread is being driven by sex itself or merely close contact. Anyone can be infected through close contact with a sick person — or their clothing or bedsheets.

“By nature, sexual activity involves intimate contact, which one would expect to increase the likelihood of transmission, whatever a person’s sexual orientation and irrespective of the mode of transmission,” said Mike Skinner, a virologist at Imperial College London.

On Monday, the European Centre for Disease Prevention and Control Director Andrea Ammon said “the likelihood of further spread of the virus through close contact, for example during sexual activities among persons with multiple sexual partners, is considered to be high.”

To date, WHO has recorded more than 90 cases of monkeypox in a dozen countries including Britain, Spain, Israel, France, Switzerland, the U.S. and Australia. On Monday, Denmark announced its first case, Portugal revised its total upwards to 37 and Italy reported one further infection.

Germany has four confirmed cases linked to exposure at “party events … where sexual activity took place” in Spain’s Canary Islands and in Berlin, according to a government report to lawmakers obtained by the AP.

Madrid’s senior health official said Monday that the Spanish capital had 30 confirmed cases. Enrique Ruiz Escudero said authorities are investigating possible links between a recent Gay Pride event in the Canary Islands, which drew some 80,000 people, and cases at a Madrid sauna.

The monkeypox cases so far have been mild, with no deaths reported. Typically, the virus causes fever, chills, rash and lesions on the face or genitals. Most people recover within several weeks without requiring hospitalization.

Vaccines against smallpox, a related disease, are effective in preventing monkeypox and some antiviral drugs are being developed. In recent years, the illness has been fatal in up to 6% of infections.

Heymann chaired an urgent meeting of WHO’s advisory group on infectious disease threats on Friday to assess the outbreak and said there was no evidence to suggest that monkeypox had mutated into a more infectious form.

The U.N. agency said the outbreak is “a highly unusual event” and said the fact that cases are being seen in so many different countries suggests the virus may have been silently spreading for some time. The agency’s Europe director warned that festivals and parties could accelerate spread.

Still, at a public session on Monday, WHO officials described the outbreak as “containable” and cautioned against stigmatizing affected groups, saying the disease can infect anyone.

Authorities in Britain, Spain and Portugal have said most of the cases identified so far were in young men whose infections were picked up when they sought help for lesions at sexual health clinics.

Heymann, who is also a professor of infectious diseases at the London School of Hygiene and Tropical Medicine, said the monkeypox outbreak was likely a random event that might be traceable to a single infection.

“It’s very possible there was somebody who got infected, developed lesions on the genitals, hands or somewhere else, and then spread it to others when there was sexual or close, physical contact,” Heymann hypothesized. “And then there were these international events that seeded the outbreak around the world, into the U.S. and other European countries.”

He emphasized that the disease was unlikely to trigger widespread transmission.

“This is not COVID,” he said. “We need to slow it down, but it does not spread in the air and we have vaccines to protect against it.”

Heymann said studies should be conducted rapidly to determine if monkeypox could be spread by people without symptoms and that populations at risk of the disease should take precautions to protect themselves.

— Barry Hatton in Lisbon, Portugal, and Frank Jordans and Kirsten Grieshaber in Berlin, contributed to this report.

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