The global monkeypox outbreak is primarily being driven by sex between men, according to the first major peer-reviewed paper to analyze a large set of cases of the virus.

The outbreak, which epidemiologists believe initially began in mid-spring gatherings of gay and bisexual men in Europe, has since alarmed such experts by ballooning to nearly 16,000 cases worldwide.

Now infectious disease specialists are developing an increasingly refined understanding of the predominant conduits of monkeypox transmission, as well as the typical disease course patterns.

“These data point clearly to the fact that infections are so far almost exclusively occurring among men who have sex with men,” said Jennifer Nuzzo, an epidemiologist at Brown University, of the new study, which was published Thursday in the New England Journal of Medicine. “And the clinical presentation of these infections suggest that sexual transmission, not just close physical contact, may be helping spread the virus among this population.”

“This large, multicountry study provides the most complete set of clinical and demographic data on monkeypox cases occurring outside endemic areas,” said Nuzzo, who was not involved with the study.

No one has died of monkeypox infection outside of Africa during this outbreak. And for many people, the disease is relatively mild and resolves on its own in a few weeks without any need for medical intervention. However, the new paper reports that monkeypox can cause pain so intense that a substantial proportion of people with the virus require hospitalization for pain management.

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