At least 19 people in three Midwestern states have contracted a disease related to smallpox, marking the first outbreak of the life-threatening illness in the United States, federal heath officials said yesterday.

The federal Centers for Disease Control and Prevention, concerned that the illness could spread, issued a nationwide alert to doctors and public health officials to be on the lookout for more cases.

"We have an outbreak," said James Hughes, director of the CDC's National Center for Infectious Diseases in Atlanta. "I'd like to keep it relatively small. I don't want any more cases. We're doing everything we can to try to contain this."

The disease, known as monkeypox, usually only occurs in central and western Africa. It is caused by a virus known as an orthopox virus, which is the family of viruses that includes the smallpox virus, one of the most dangerous diseases known to man and a feared biological weapon.

Officials said there was no indication that bioterrorism was involved. The disease was apparently spread by rodents known as prairie dogs, which have become popular as pets. The animals may have acquired the infection from another creature, known as a Gambian giant rat, sold by the same dealer of exotic animals, officials said.

The monkeypox virus causes symptoms that are very similar to smallpox -- fever, headache, cough and an extremely painful rash of pus-filled sores that spreads across the body.

While much about the monkeypox virus is unclear, it does not seem to be as deadly as smallpox. Authorities estimate that monkeypox has a mortality rate of between 1 percent and 10 percent, compared with a mortality rate of about 30 percent for smallpox.

The monkeypox virus is believed to spread through physical contact with a sick person or infected animal, or through infected body fluids, although health officials said it apparently is not as easily spread as smallpox, which is highly infectious.

Monkeypox is untreatable, although there is some indication that an antiviral drug may have some usefulness. Because the disease has never been seen before in this part of the world, the severity of the threat is not completely clear. All patients and infected animals have been isolated -- and pet shops and one house where the prairie dogs lived have been quarantined -- to prevent spread of the disease.

The smallpox vaccine is believed to be protective against the monkeypox virus. The federal government recently launched a campaign to vaccinate thousands of emergency workers against smallpox so the country would be prepared in the event of a bioterrorist attack.

"This is an unusual event. As far as we can tell, there's never been a human or animal illness in the community setting in the Western hemisphere by a virus that is either a monkeypox virus or a very close variant of the monkeypox virus," said Hughes, who held a hastily arranged telebriefing last evening to announce the outbreak after CDC scientists confirmed that a monkeypox virus or one very close to it was involved.

"We've got a disease that's not been seen before in the Western Hemisphere, so it's prudent to take it very seriously," Hughes said in a telephone interview after the briefing.

Of the 19 cases reported so far, four of the victims have been hospitalized; none has died, Hughes said.

The outbreak came to light on May 16, when a 3 1/2-year-old child became ill, according to John Melski, who treated the child at the Marshfield Clinic in Marshfield, Wis.

The child's parents had bought two prairie dogs as a Mother's Day present for the child's mother. Both the mother and father subsequently became ill as well, although all appear to have recovered.

Officials determined that the prairie dogs had been purchased from a Villa Park, Ill., exotic pet dealer, who also became ill. The dealer also had a Gambian rat, which was ill. It is believed that animal passed the virus to the prairie dogs the dealer was selling.

The dealer sold the animals to SK Exotics, a Milwaukee pet distributor, which then sold the apparently infected prairie dogs to two pet stores in Milwaukee and at a "pet swap" in northern Wisconsin.

Most of the rest of the cases have been reported in the Milwaukee area, and are believed to have involved people who either worked at the stores or who handled the animals in the stores. Seventeen of the cases occurred in Milwaukee, with one case each having been reported in Illinois and Indiana.

Melski and his colleagues at the Marshfield Clinic contacted state health officials when they identified what appeared to be an orthopox virus in the sick family. State health officials then contacted the CDC, which confirmed the involvement of a monkeypox-like virus yesterday, prompting the nationwide alert and telebriefing.

The state of Wisconsin has temporarily banned the sale of prairie dogs.

"The full impact is hard to predict," said Seth Foldy, Milwaukee's health commissioner. "Our goal would be to isolate and eliminate the virus from both human and animal populations to the best of our ability. We do not know if it is the kind of agent that would or could thrive in North America, and we're not very interested in finding out that it is."

The last time a new infectious disease arrived in the United States was in 1999, when the West Nile virus was first reported. That disease, which is transmitted by mosquitoes, has since spread nationwide.

Further tests are planned to confirm the identity of the pox-like virus.

The outbreak comes as the global epidemic of severe acute respiratory syndrome (SARS) appears to be coming under control.

"This is yet another reminder of why it's important to learn as much as you can about diseases that occur in faraway places," Hughes said.