For decades, a few scientists have voiced concerns that the monkeypox virus could have become better at infecting people—ironically because we eradicated its relative, smallpox, in the late 1970s. The smallpox vaccine incidentally protected against monkeypox. And when new generations were born into a world without either smallpox or smallpox-vaccination campaigns, they grew up vulnerable to monkeypox. In the Democratic Republic of Congo, this dwindling immunity meant that monkeypox infections increased 20-fold in the three decades after smallpox vanished, as Rimoin showed in 2010. That gives the virus more chances to evolve into a more transmissible pathogen in humans. To date, its R0—the average number of people who catch the disease from one infected person—has been less than 1, which means that outbreaks naturally peter out. But it could eventually evolve above that threshold, and cause more protracted epidemics, as Bergstrom simulated in 2003. “We saw monkeypox as a ticking time bomb,” he told me.

This possibility casts a cloud of uncertainty over the current unusual outbreaks, which everyone I spoke with is concerned about. Are they the work of a new and more transmissible strain of monkeypox? Or are they simply the result of people traveling more after global COVID restrictions were lifted? Or could they be due to something else entirely? So far, the cases are more numerous than a normal monkeypox outbreak, but not so numerous as to suggest a radically different virus, Inglesby told me. But he also doesn’t have a clear explanation for the outbreak’s unusual patterns—nor does anyone else.

Answers should come quickly, though. Within days, scientists should have sequenced the viruses from the current outbreaks, which will show whether they harbor mutations that might have changed their properties. Within weeks, European epidemiologists should have a clearer idea of how the existing cases began, and whether there are connections between them. As for the U.S., “there are currently no additional suspected cases” beyond the individual in Massachusetts, McCollum told me. But given the numbers in Europe, she’s standing by for more.