Even as cases rise, genetic analysis suggests that the virus has been silently circulating in people since 2018.

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.

Genetic analysis suggests that although the monkeypox virus is rapidly spreading in the open, it has been silently circulating in people for years.

If the virus has adapted to include people as hosts, monkeypox outbreaks could become more frequent and more difficult to contain.

That carries the risk that monkeypox could spill over from infected people into animals — most likely rodents — in countries outside Africa, which has struggled with that problem for decades.

The virus may persist in infected animals, sporadically triggering new infections in people.

The longer it takes to contain the virus, the higher the odds that it will find a permanent new home in people or animals, Dr.

This enzyme, first discovered by researchers studying H.I.V., is a so-called host defense factor — an immune-system weapon that animals and people use to disarm viruses like monkeypox.

The rapid accumulation of mutations, characteristic of the enzyme since 2018, suggests that monkeypox may have switched to people as hosts around then, Dr.

Most people have mild symptoms and recover within weeks, but the virus can be fatal in a small percentage of cases?

While Covid-19 is a tiny RNA virus that can spread through aerosols, monkeypox is a larger DNA virus that is transmitted mostly through close physical contact and has a much smaller mutation rate than RNA viruses.

Adetifa said, “we suspect some underreporting because there’s been relatively low awareness and maybe low perceived risk of monkeypox.” Nigeria is stepping up its surveillance of monkeypox, and case numbers may rise as more people become aware of the virus, he added.

Britain’s experience indicates how complicated it can be to trace contacts of a virus that may be sexually transmitted, especially in cases where infected people have had multiple anonymous partners.

As deforestation and urbanization drive people and animals into closer quarters, more viruses may make the jump to human hosts.

In a 2003 outbreak in the United States, a shipment of Gambian pouched rats imported from Africa transmitted monkeypox to prairie dogs, which then infected 71 Americans.

But officials did not find signs of the virus in animals in the United States once the spate of cases had ended.