Persistent use of methamphetamine is the single biggest risk factor for HIV seroconversion among gay and bisexual men, according to a new study published in the Journal of Acquired Immune Deficiency Syndromes. Using subjects from across the U.S., including Puerto Rico and Guam, researchers found that over 12 months of follow-up, 14% of study participants reported using meth, and 2.5% tested positive for HIV (115 new infections). More than a third of study participants (36%) seroconverting for HIV were persistent users of methamphetamine.