HIV among Gay, Bisexual and Other Men Who Have Sex with Men (MSM)| Topics | CDC HIV/AIDS Gay, bisexual, and other men who have sex with men (MSM)1 represent approximately 2% of the US population, yet are the population most severely affected by HIV and are the only risk group in which new HIV infections have been increasing steadily since the early 1990s. In 2006, MSM accounted for more than half (53%) of all new HIV infections in the United States, and MSM with a history of injection drug use (MSM-IDU) accounted for an additional 4% of new infections. At the end of 2006, more than half (53%) of all people living with HIV in the United States were MSM or MSM-IDU. Since the beginning of the US epidemic, MSM have consistently represented the largest percentage of persons diagnosed with AIDS and persons with an AIDS diagnosis who have died. Only 2% of the population but account for over half of all HIV/AIDS infections. If it isn't their high risk sexual lifestyle, what is it?