LONDON (AP) — A leading adviser to the World Health Organization described the unprecedented outbreak of the rare disease monkeypox in developed countries as “a random event” that might be explained by risky sexual behaviour at two recent mass events in Europe.

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.

A German government report to lawmakers, obtained by the AP, said it expected to see further cases and that the risk of catching monkeypox “mainly appears to lie with sexual contacts among men.”

The four confirmed cases in Germany have been linked to exposure at “party events including on Gran Canaria and in Berlin, where sexual activity took place,” it said.

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

Madrid’s senior health official said on Monday that the Spanish capital has recorded 30 confirmed cases so far. 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.

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

Monkeypox typically causes fever, chills, rash, and lesions on the face or genitals. It can be spread through close contact with an infected person or their clothing or bedsheets, but sexual transmission has not yet been documented. Most people recover from the disease within several weeks without requiring hospitalisation. Vaccines against smallpox, a related disease, are also effective in preventing monkeypox and some antiviral drugs are being developed.

In recent years, the disease has been fatal in up to six per cent of infections, but no deaths have been reported among the current cases. WHO said confirmed cases have so far been the less severe West African group of monkeypox viruses and appeared to be linked to a virus that was first detected in exported cases from Nigeria to Britain, Israel and Singapore in 2018-2019.

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 disease may have been silently spreading for some time. The agency’s Europe director warned that as summer begins across the continent, mass gatherings, festivals and parties could accelerate the spread of monkeypox.

Other scientists have pointed out that it will be difficult to disentangle whether it is sex itself or the close contact related to sex that has driven the recent spread of monkeypox across Europe.

“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 Sunday, the chief medical adviser of Britain’s Health Security Agency, Dr Susan Hopkins, said she expected more monkeypox cases to be identified in the country “on a daily basis.”

U.K. officials have said “a notable proportion” of the cases in Britain and Europe have been in young men with no history of travel to Africa and who are gay, bisexual or have sex with men. Authorities in Portugal and Spain also said their cases were in men who mostly had sex with other men and 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 hypothesised. “And then there were these international events that seeded the outbreak around the world, into the US and other European countries.”

He emphasised 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.