The US could face further outbreaks of dangerous monkeypox if the virus has gained a foothold among native animals, say experts.
The disease covers the body with lesions (courtesy of Leo Lanoie)
The illness, from the same family as smallpox, is endemic in parts of Africa, but 81 cases were reported last year in six US states.
Writing in The Lancet Infectious Diseases, researchers from Stanford University say that it could return.
Some scientists say that, in theory, it could be used as a bioweapon.
It is thought that monkeypox was carried into the US by rodents imported as exotic pets.
It is extremely contagious among rodents, and fears now centre on whether or not the virus has managed to spread into native rat and mouse populations.
If so, say Drs Daniel Di Giulio and Paul Eckburg, it could form a "reservoir" of the virus which could provide the source of further outbreaks.
In Africa, death rates during outbreaks have varied from 1.5% to 17%, although none of the 81 people infected in the US succumbed to the infection.
The first case was recorded in 1970, shortly after the eradication of smallpox was completed in what is now the Democratic Republic of Congo.
Since then there have been only sporadic cases in rainforest areas in central and western Africa, and an organised surveillance programme was halted in 1986.
A typical monkeypox infection first shows itself as a rash of lesions between two and five millimetres in diameter, spreading outwards.
The best known treatment is the administration of the smallpox vaccine.
While its spread from human to human is not completely understood, doctors believe that, unlike smallpox, it currently finds it hard to jump from person to person.
While a five-link "chain" of human to human transmission has been documented, experts say that it is highly unlikely it could sustain itself permanently among humans - and that the biggest risk of exposure would be contact with an infected animal.
The authors of the Lancet paper call for extra vigilance against the infection.
They write: "Although wild-type monkeypox virus has a very low potential for use as an agent of bioterrorism, how readily the virus can be genetically manipulated to exhibit greater virulence or transmissability for such use is less clear."
Exotic rodents have been blamed for spreading monkeypox
The authors point to an experiment in which a related virus was transformed into a killer by a simple genetic manipulation during an attempt to produce a contraceptive vaccine.
They write: "We can no longer afford to ignore uncommon, geographically restricted or seemingly conquered infectious agents.
"If the US is fortunate enough to have avoided the establishment of a new endemic zoonosis [infection which jumps from animal to human] this time, the next outbreak caused by an emerging agent, wherever it occurs, may prove very different."