A vicious skin infection resistant to all but the most powerful antibiotics has jumped out of New York City hospitals and onto the streets.

The "superbug," as health officials refer to it, can cause anything from reddening of the skin, to abscesses, tissue loss, amputation or even death in severe cases, doctors said.

For decades confined to hospitals, where it preyed on patients and built up immunity to antibiotics, the bug - known officially as Methicillin Resistant Staphylococcus Aureus or MRSA - has also grown in strength.

"Usually with infections you need a break in the skin to pass it," said Dr. Howard Grossman, who has a private practice in Chelsea.

"Not with this. It gets through unbroken skin with casual contact."

The city Department of Health first detected the infections outside hospitals early last year, according to health officials.

Doctors at some clinics, such as the Callen-Lorde Community Health Center in Chelsea, are seeing one new case a week, compared with one every two months when the infection first cropped up last year.

"This is something we should be concerned about," said Dr. Dawn Harbatkin, the center's medical director.

Dr. Brian Saltzman of Beth Israel, who has just completed a study of the spread of MRSA outside hospitals, said, "We are seeing very impressive, very large, very difficult-to-treat skin abscesses."

The Department of Health is tracking the outbreak here but declined to provide the number of cases it has found.

Last month, Steven, who asked that his last name not be published, developed what he thought was a pimple on his leg, but it soon grew painful and larger.

Doctors lanced the boil that formed and began antibiotics, but the infection failed to respond and starting growing toward Steven's groin.

"The fact it wasn't responding [to drugs] and it was moving up that way was terrifying," he said. "It was eating up tissue."

After a lengthy hospital stay and five antibiotics - some administered intravenously and one, Zyvox, administered orally at $100 per tablet - the infection started to abate.

Doctors told Steven they believed he contracted it at the gym. Keith, who lives in West New York, N.J., has been battling MRSA for months, with the infection cropping up on his legs, then his face, then back on his legs. The doctor treating him says the infection has "colonized" inside him. He believes Keith contracted it from a friend.

In New York City hospitals, about 50 percent of infections are now resistant to some kind of antibiotic, as opposed to 10 percent a decade ago, according to several local infectious-disease specialists.

According to an Institute of Medicine report last year, 80,000 people die each year in the United States from hospital-acquired infections.

The city is not aware of anyone dying of MRSA acquired outside hospitals.

There are now three antibiotics left that can attack MRSA: vancomysin, daptomycin and linezolid. But those antibiotics are beginning to lose their potency against the bug.