Worse, on June 3 the CDC announced it had found genetic evidence of U.S. pox cases that predated the first cases in Europe from May. Doctors may not have noticed or reported these earlier cases, at first, owing to the similarity between pox symptoms and the symptoms of some common sexually-transmitted diseases such as herpes.

There was some speculation that the earlier U.S. cases were part of a totally separate outbreak that just happened to overlap with the May outbreak. Isidro and her team sequenced 15 samples taken from current pox patients and concluded that, no, there’s just one big outbreak. “All outbreak MPX strains sequenced so far tightly cluster together, suggesting that the ongoing outbreak has a single origin,” they wrote, using the scientific acronym for monkeypox.

It’s less clear exactly when the current outbreak really began. According to Isidro and company, the virus may have been circulating outside of endemic countries long before officials finally noticed the infections and sounded the alarm. The virus potentially traveled beyond Africa in animals such as pet rodents, and spread from animal to animal before finally jumping to a human host and triggering the current outbreak some time prior to May, the geneticists wrote.