The ongoing monkeypox outbreak could have stemmed from sexual activity at raves in Europe, according to a World Health Organization expert.

WHO convened an emergency meeting this weekend via video conference to look at the virus, identify those most at risk and study its transmission. The organization will hold a second global meeting on monkeypox next week to more thoroughly study the risks and treatments available to fight the virus.

The outbreak of the monkeypox virus in the US and Europe is primarily spreading through sex-among-men with about 200 confirmed and suspected cases across at least a dozen countries, World Health Organization officials said Monday.

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

The chairman of the World Health Organization Emergency Committee, Professor David L. Heymann, said that the unusual spread could have originated from sexual encounters at recent gatherings in Belgium and Spain.

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

“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 said. “And then there were these international events that seeded the outbreak around the world, into the US and other European countries.”

WHO has confirmed about 90 cases of monkeypox in a dozen countries including the US, Germany, France, Israel and Denmark. Belgium just introduced a mandatory 21-day quarantine for monkeypox patients.

The U.S. has already stockpiled smallpox vaccines, which can protect against monkeypox, to vaccinate the entire population of the U.S., according to Raymond James analysts.

Meanwhile, the growing concern about the emergence of monkeypox in Europe and the U.S. has sent shares of several vaccines and drug companies soaring. Several companies that have developed smallpox vaccines or therapeutics have seen their share prices jump since the outbreak.

Some of these therapeutic companies include GeoVax Labs Inc, Emergent Biosolutions Inc and Inovio Pharmaceuticals Inc according to MarketWatch