Recent research suggests that unprotected sex between gay men—especially if they are HIV positive—is promoting hepatitis C transmission in major urban areas across the globe.

Daniel Fierer, MD, of The Mount Sinai Hospital in New York, says many of the HIV-positive gay men he treats are shocked to find themselves suddenly coinfected with HCV, often after what he calls a “bad weekend.”

Unlike HIV, HCV doesn’t seem to be transmitted among monogamous heterosexual couples. The HCV risk for gay men often lies in traumatic sex practices common to crystal meth–fueled sex between men. Group sex, fisting, use of toys and rough anal sex—anything that produces even microscopic amounts of blood—appear to put both the top  (insertive) and bottom (receptive) partners at risk for hep C. Having HIV, as well as having other sexually transmitted infections, sharply raises the risk.

You don’t have to be celibate, though.

“It’s such an old message, it’s ​almost hackneyed,” Fierer says: “Wear a condom.”