At least 28 people have been infected with "monkeypox", a disease related to smallpox and usually found in central and west African rain forests, although none of the American patients has died.
The infections are thought to have been passed on by prairie dogs, wild rodents that live in holes on the western plains of the US and that have become popular in America's exotic pet market recently.
The symptoms of monkeypox include fever, headaches, swollen lymph nodes, chills, sweats and a dry cough. Ten days after contracting the disease, patients break out in a rash of blistering pimples filled with pus, which open and produce scabs.
The patients range in age from four to 48, and come from Wisconsin, Illinois and Indiana. Health officials expect the numbers to rise now that the public is on alert for the symptoms. All those infected had recent close contact with prairie dogs.
It does not appear that anyone contracted the virus from another person, doctors say.
The disease has a mortality rate of up to 10%, with young children more likely to perish. However, with better standards of nourishment and medical provision in America, the virus may prove less lethal.
Officials believe the disease may have been carried to the US by Gambian giant pouched rats, which then passed it on to the prairie dogs. A pet shop in a Chicago suburb has been quarantined after the person in Illinois suspected of having the disease said he had been in contact with prairie dogs there.
The shop sold both the prairie dogs and the Gambian rat to an animal distributor in Wisconsin, which in turn sold the prairie dogs to pet shops in the Milwaukee area. State and federal authorities are trying to trace about 200 animals that were distributed in 15 states by the dealer.
One of the first cases to be diagnosed was a four-year-old girl who was bitten on the finger by her new pet prairie dog in mid-May. Her parents later came down with similar symptoms. Clinical tests showed a pox-like virus, with cultures that matched both her mother and the prairie dog.
"Right then we knew we had something interesting," Dr Kurt Reed, an infectious-disease pathologist told the new York Times. "We do lots and lots of virus cultures. This was very unusual. There's nothing really in the literature about prairie dogs having pox viruses."
When Dr Reed contacted the state health department, he found that similar cases had been discovered in the area. "We feel pretty certain now that it is monkeypox," said Dr Stephen Ostroff, the deputy director for the national centres for infectious diseases.
Monkeypox is the latest in a series of new diseases that have recently surfaced in America. Four years, ago west Nile fever arrived in the country, carried by mosquitoes in a shipment of tyres.
· The disease has never before been reported in the western hemisphere. It is usually found in remote areas of central and west Africa.
· Monkeypox is related to the smallpox virus, and smallpox vaccinations protect against it.
· Up to 10% of cases die. Children are most at risk.
· It is usually transmitted to people from squirrels and primates, through a bite or contact with the animal's blood. The US cases are believed to have had "close contact" with prairie dogs.