Until recently, these hard-to-treat cases were seen only in hospitals and other health-care settings where they can spread to patients with open wounds or tubes and cause serious complications. Now doctors are seeing resistant strains among inmates, children and athletes.

Researchers at the Centers for Disease Control and Prevention suspected that those outside infections might just be leaking out of hospitals rather than emerging from the general population. But their study in Baltimore, the Atlanta area and Minnesota proved that theory wrong.

Overall, they found 17 percent of drug-resistant staph infections were caught in the community and did not have any apparent links to health-care settings.

"Close to one-fifth of what used to be a hospital-specific problem is now a community problem. And that's a large number," said the CDC's Dr. Scott K. Fridkin. "We didn't think it would be anywhere near that high when we started the study."

Their findings are published in New England Journal of Medicine.

Staph bacteria are a common cause of skin infections. Healthy people may carry the bacteria on their skin and in their noses. When infections occur, they are mostly pimples and boils, but the germ can cause serious surgical wound infections, bloodstream infections and pneumonia.

Three-quarters of the community-acquired cases in the CDC study were skin infections, but 23 percent of the cases were serious enough to require hospitalization.

Staph bacteria resistant to the penicillin drug family are called methicillin-resistant staphylococcus aureus, or MRSA.

The CDC researchers checked up to two years of lab reports for drug-resistant staph. More than 80 percent of the 12,553 cases were excluded because the patients had been hospitalized, had a history of surgery or dialysis or had another risk factor.

Children at highest risk

About 17 percent overall, or 2,107 cases, were determined to be community-acquired staph. The rate was 20 percent in Atlanta, 12 percent in Minnesota and 8 percent in Baltimore.

"When they got out in the community, it was felt these strains weren't strong enough to make it on their own. That no longer appears to be the case," said Dr. Henry F. Chambers of the University of California at San Francisco, who wrote an accompanying editorial.

The CDC research found that children under 2 were at higher risk, which could be because children get more cuts and scrapes. Blacks in Atlanta were found to be at higher risk than whites. In cases confirmed through interviews, half were in people who shared a bedroom, and only about one in 10 were in day care.

Fridkin said the study might have underestimated drug-resistant staph out in the community because not all cases are sent to labs for analysis.

Philip Tierno, director of clinical microbiology at NYU Medical Center, said people can help prevent staph infections by washing their hands, using an antiseptic and a bandage on all cuts and scrapes, and avoiding the sharing of towels, razors, clothing and athletic equipment.