SAN FRANCISCO (Reuters) - A drug-resistant strain of potentially deadly bacteria has moved beyond the borders of U.S. hospitals and is being transmitted among gay men during sex, researchers said on Monday.

They said methicillin-resistant Staphylococcus aureus, or MRSA, is beginning to appear outside hospitals in San Francisco, Boston, New York and Los Angeles.

Sexually active gay men in San Francisco are 13 times more likely to be infected than their heterosexual neighbors, the researchers reported in the Annals of Internal Medicine.

"Once this reaches the general population, it will be truly unstoppable," said Binh Diep, a researcher at the University of California, San Francisco who led the study. "That's why we're trying to spread the message of prevention."

According to chemical analyses, bacteria are spreading among the gay communities of San Francisco and Boston, the researchers said.

"We think that it's spread through sexual activity," Diep said.

This superbug can cause life-threatening and disfiguring infections and can often only be treated with expensive, intravenous antibiotics.

It killed about 19,000 Americans in 2005, most of them in hospitals, according to a report published in October in the Journal of the American Medical Association.

About 30 percent of all people carry ordinary staph chronically. It can be passed by touching other people or by depositing the bacteria on surfaces or objects.

The bacteria can cause deep-tissue infections if they enter the body through a wound in the skin.

Of those people who carry staph, most carry it in their noses but community-based MRSA also can live in and around the anus and is passed between sexual partners.

Incidence of MRSA is rising along with the resurgence of syphilis, rectal gonorrhea, and new HIV infections partly because of changes in beliefs about the severity of HIV and an increase in risky behaviors, such as illicit drug use and having sex that abrades the skin, Diep's team wrote.

"Your likelihood of contracting each of these diseases increases with the number of sexual partners that you have," Diep said. "The same can probably be said for MRSA."

Staph infections often look like raised red dots on the skin. Left untreated, the areas can swell and fill with pus.

The best way to avoid infection is by washing the hands or genitals with soap and water, Diep said.