In fact, a preliminary genetic analysis from University of Edinburgh scientists suggests that the evolution of this monkeypox lineage suddenly accelerated sometime between 2017 and 2022. Poxviruses tend to accumulate mutations at a fairly slow rate of one or two a year, but the genomes from 2022 have a whopping 47 mutations. Intriguingly, almost all of the changes to the genetic code are TC to TT or GA to AA. This is unlikely to have happened through random copying error; instead it resembles the signature of an immune-system mechanism—found in both humans and animals—that introduces mutations in an attempt to disable the virus. This signature is seen in many common viruses, including SARS-CoV-2, notes Nicolas Gillet, a biologist at the University of Namur who has studied this defense mechanism. You can think of most of the mutations as “scars” from battling with the host immune system, says Richard Neher, a biologist at the University of Basel, though it’s impossible to say whether any could also be adaptive. In any case, monkeypox seems to have found a new host since 2017: either humans directly or another animal that then spread the virus to humans.

The lack of attention to monkeypox means basic questions—such as which animal or animals in fact spread the virus—remain unanswered. Despite the name, monkeypox is more commonly found in rodents, though it can infect a wide range of species, including primates and rabbits. When it comes to the virus’s natural animal reservoir, “we don’t know,” says Boghuma Titanji, an infectious-disease doctor at Emory. In addition to the Nigeria outbreak that began in 2017, a separate outbreak of a more severe form of monkeypox has been intensifying around the Democratic Republic of Congo, where the virus has long circulated. The Congo has seen 1,200 cases and 58 deaths this year alone.