When the first monkeypox cases were identified in early May, European health officials were stumped. The virus was not known to spread easily among people, let alone infect dozens — and soon hundreds — of young men.
The origins of the outbreak are now becoming clearer. Genetic analysis suggests that although the monkeypox virus is rapidly spreading in the open, it has been silently circulating in people for years.
Health officials have already identified two versions of monkeypox among American patients, suggesting at least two separate chains of transmission. Researchers in several countries have found cases with no known source of infection, indicating undetected community spread. And one research team argued last month that monkeypox had already crossed a threshold into sustainable person-to-person transmission.
The genetic information available so far indicated that, at some point in the last few years, the virus became better at spreading between people, said Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.
“Genomic patterns would suggest this occurred around 2018,” Dr. Bedford said.
If the virus has adapted to include people as hosts, monkeypox outbreaks could become more frequent and more difficult to contain.
