A leading adviser to the World Health Organization speculated that the unprecedented outbreak of monkeypox may be linked to sexual activity at two raves in Europe. The WHO has recorded more than 90 cases of monkeypox in a dozen countries, including the U.S., where there is one confirmed case and four likely cases. Dr. David Heymann told the Associated Press that the current leading theory is that the outbreak has been “amplified” by sexual activity at raves in Belgium and Spain. Scientists expect that it will be difficult to distinguish whether the spread is driven by sex itself or the close contact inherent in sexual activity. Health officials are reporting that most known cases in Europe are among men who have sex with men, but are emphasizing that the disease can affect anyone irrespective of sexual orientation. The WHO has described the current outbreak as “a highly unusual event,” though one that is “containable.”