A new, highly drug-resistant strain of the “flesh-eating” MRSA bacteria is being spread among gay men in San Francisco and Boston, researchers reported on Monday.

In a study published online by the journal Annals of Internal Medicine, the bacteria seemed to be spread most easily through anal intercourse but also through casual skin-to-skin contact and touching contaminated surfaces.

The authors warned that unless microbiology laboratories were able to identify the strain and doctors prescribed the proper antibiotic therapy, the infection could soon spread among other groups and become a wider threat.

The new strain seems to have “spread rapidly” in gay populations in San Francisco and Boston, the researchers wrote, and “has the potential for rapid, nationwide dissemination” among gay men.