Still, for monkeypox to be detected in people with no apparent connection to one another suggests that the virus might have been spreading silently — a fact that Andrea McCollum, an epidemiologist who heads the US Centers for Disease Control and Prevention poxvirus team calls “deeply concerning”.

Unlike SARS-CoV-2, which can spread asymptomatically, monkeypox does not usually go unnoticed when it infects a person, in part because of the skin lesions it causes. If monkeypox could spread asymptomatically, it would be especially troubling because it would make the virus harder to track, McCollum says.

Another puzzle is why almost all of the case clusters include men aged 20–50, many of whom are gay, bisexual and have sex with men (GBMSM). Although monkeypox isn’t known to be sexually transmitted, sexual activity certainly constitutes close contact, Rimoin says. The most likely explanation for this unexpected pattern of transmission, MacIntyre says, is that the virus was coincidentally introduced into a GBMSM community, and the virus has continued circulating there. Scientists will have a better idea of the origin of the outbreaks and the risk factors for infection once an epidemiological investigation is complete, which can take weeks and involves rigorous contact tracing.