With infections that outsmart powerful antibiotics on the rise, doctors and public health officials have long worried that they might face an outbreak of resistant bacteria that threaten large numbers of people. Now they've found it -- in Los Angeles County.

The large, painful skin infections started turning up early last fall among local gay men, then appeared with increasing frequency over the ensuing weeks and months. Although doctors found the symptoms alarming, it took a while to confirm a connection between these cases. Now they know they're facing an emerging epidemic of drug-resistant Staphylococcus aureus or, more simply, staph.

Although the outbreak seems confined primarily to gay men, doctors say at least one woman contracted the infection, probably from a male sex partner. Because they still know so little about the extent of the outbreak, they can't predict how many people it may eventually affect.

The infection, which causes nasty-looking boils, deep abscesses and widespread surrounding inflammation, has proved impervious to common antibiotics. Although it appears to be spread primarily by skin-to-skin contact, including sex, its origins and precise mode of transmission remain a mystery. Doctors treating it caution that it could also be contracted at health clubs, steam rooms and other warm, moist environments.

"The concern is this organism could spread to and cause disease in the community at large," said Dr. Peter Ruane, an infectious disease specialist in Los Angeles. "It seems to be able to attack normal skin in healthy people."

County health officials, with assistance from the federal Centers for Disease Control and Prevention, have begun a public health investigation to understand how the infection is spread, determine who else may be affected and develop strategies to contain it.

"Primary-care doctors and ER doctors need to know about this so that when they encounter these infections, they can prescribe a drug that's active against resistant staph," Ruane said. Treatment typically involves draining the abscess, culturing the bacteria to see what they are resistant to, then prescribing appropriate antibiotics.

Cases of drug-resistant staph, long recognized as a problem in nursing homes and hospitals, have increasingly cropped up in clusters outside those settings. But the current outbreak marks the first time resistant staph has been reported in the gay community, which doctors say gives it the potential to spread widely and quickly because many of their gay patients frequent circuit parties and bathhouses, engaging in sex with multiple partners.

Medical treatment has resolved most of the infections, with more aggressive cases requiring prolonged hospitalization and intravenous antibiotics. In some cases, the infections have taken up to a month to clear up. However, because staph isn't spread through the air like the flu, it would be less likely than those infections to cause widespread disease and fill emergency rooms.

Still, although no one has died, doctors are grappling with the daunting prospect that the bug will outsmart additional antibiotics, further narrowing treatment options. They know that in cases where the infection spreads to the blood, it can prove fatal if antibiotics fail or it goes untreated.

Dr. Gary Cohan, managing director of Pacific Oaks Medical Group in Beverly Hills, one of the nation's largest private AIDS practices, says he used to see one antibiotic-resistant staph case a year among hospitalized AIDS patients. Now, two patients a week come to the office with "abscesses filled with a viciously resistant staph."

"It's an evolving story," said Ruane, who in September began noticing an increasing number of the aggressive staph infections in his gay patients. "The aggressiveness of this took us aback."

Because Staphylococcus aureus lives on the skin's surface, usually existing harmlessly in the nose, armpits and groin, infections typically start in a cut or other opening. But the infections seen in local gay men -- the majority with well-controlled HIV or AIDS, but many others in good health -- took hold in unbroken skin. Doctors also noticed that the painful boils, and abscesses were in rather unusual places: the legs, buttocks, penis and scrotum, as well as hands and face.

After comparing notes with several colleagues also seeing unusual skin infections in gay patients, Ruane said, "we felt we were looking at some form of outbreak." Ruane then notified county health officials.

Ruane, an AIDS specialist and director of research at Tower Infectious Disease Medical Associates in Los Angeles, is aware of 40 cases among his and colleagues' patients. Others may go unrecognized because some people drain their own boils and never seek medical care.

He sent specimens to Cedars-Sinai Medical Center's microbiology lab, where they were analyzed by a scientist, Margie A. Morgan, and her boss, Dr. Steven Nichols. Using a scientific technique called molecular fingerprinting, the two found, as suspected, the cases involved a genetically identical strain of resistant staph. They also found that the strain contains a powerful toxin called Panton-Valentine leukocidin seen in resistant staph outbreaks in France and in this country. No one knows if that toxin is responsible for the microbe's ability to break through the skin, and Morgan is looking for other toxins. The county is sending samples to the CDC for further tests and to see if the same strain has been seen elsewhere.

Already, microbiologist Barry N. Kreiswirth, a researcher at the nonprofit Public Health Research Institute in Newark, N.J., has found that the strain in Los Angeles is the same one he found among 39 hospitalized AIDS patients in 1997 in New York City.

Meanwhile, local epidemiologists compared the infections among gay men to last summer's limited outbreaks of resistant staph among five newborns and among three young adult athletes. But the staph strain in those cases matched that seen in the gay patients. The epidemiologists also have seen the same strain for many months in an ongoing outbreak associated with what they will only describe as a "large institution." That outbreak remains under investigation. Although it's unusual to have the same strain in multiple locations, it may be that the current strain is gaining a local foothold, they say.

"It's only in the past couple of weeks when we got the molecular fingerprinting results that we were able to go back and connect the dots," said Dr. Elizabeth A. Bancroft, a medical epidemiologist with the Los Angeles County Health Department who is leading the investigation. All the local cases involved skin infections only; those in hospitalized patients typically involve fever and pneumonia as well as an infected surgical site or wound. In addition to alerting local doctors, she has launched a study with Ruane and the Los Angeles-based AIDS Healthcare Foundation that will try to understand the risk factors for contracting this strain. The infections, known as methicillin-resistant Staphylococcus aureus because of their resistance to the antibiotic methicillin, are the bane of modern health care because they don't respond to the cheapest and most frequently used antibiotics, including methicillin, penicillin and penicillin-like cephalosporins such as Keflex and Ceftin.

Federal figures show the percentage of staph specimens resistant to antibiotics increased from 2% in 1974 to 50% in 1997. But not until 1999 did the public-health community begin appreciating the severity of the resistance problem outside hospitals.

That's when the federal Centers for Disease Control and Prevention reported the deaths of four children in Minnesota and North Dakota whose infections didn't respond to cephalosporins. Similar outbreaks, sometimes with fatalities, have been reported among intravenous drug abusers, athletes, prisoners, Native Americans and Eskimos, whose close living conditions make them likely to share personal items such as towels.

The L.A. investigation has found that the local strain is resistant to erythromycin and the powerful fluoroquinolones, such as Cipro and Levaquin. That limits the arsenal to Bactrim, rifampin, clindamycin, and the drug of last resort, vancomycin, which is administered intravenously. Some doctors use a new antibiotic called Zyvox, although with a single course costing $1,500, they often have trouble persuading insurers to pay for it.

Bancroft stressed that until an investigation reveals how the infections are being spread, anyone with a boil or skin infection should maintain good hygiene, washing towels and anything else that comes into contact with infected areas. Any skin infection that looks particularly aggressive should be examined and cultured by a doctor. And, anyone prescribed antibiotics must complete the full course even if they start feeling better.

"The worst thing in the world is only to take a half-dose," Bancroft said, because that fosters further resistance.