Drug-resistant staphylococcus bacteria, which once threatened mainly the sick and elderly in hospitals and nursing homes, have spread beyond the institutional walls. They’re now striking young, healthy people at a growing rate.

The bacteria, mainly new strains of the types lurking in hospitals, are spread by contact with infected skin or simply by sharing towels, clothing or other personal items. The infections can be severe, especially if doctors fail to culture the bug to check for resistance.

The nationwide prevalence of drug-resistant staph infections in community settings is not known, but a study in the April 2005 issue of The New England Journal of Medicine reported 1,647 cases in Atlanta, Baltimore and parts of Minnesota alone. The study found that up to 20 percent of resistant staph cases now arise outside health-care settings.

Athletes who play contact sports may be especially vulnerable. Dozens of infections have been reported in high school and college wrestlers and football players. The U.S. Centers for Disease Control and Prevention has investigated other clusters of infection involving children at day-care centers, prison inmates, military recruits and gay men, among other groups.

The resistant infections, which almost always start on the skin, may initially look like mild spider bites, but boils and abscesses can develop.

If the problem is caught early, doctors may only have to drain the affected area, according to infectious-disease experts we consulted. But if treated late or with medications that no longer work, the bacteria can cause severe blood or lung infections.

The staph bacteria in hospitals have become resistant to most common antibiotics, although they generally respond to intravenous vancomycin (Vancocin). In the community setting, however, these infections usually can be treated with common oral medications, such as clindamycin (Cleocin) and trimethoprim-sulfamethoxazole (Bactrim, Septra).