SAN FRANCISCO — 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. […]