The Center for Strategic and International Studies (CSIS) on Thursday hosted a panel discussion focusing on the policy implications of findings published by the Lancet in a special series on HIV/AIDS and men who have sex with men (MSM), the Center for Global Health Policy's "Science Speaks" blog reports (Barton, 9/7). Chris Beyrer, a professor of international health at the Johns Hopkins Bloomberg School of Public Health and a contributor to the Lancet series, explained two factors are affecting the expansion of the HIV epidemic among MSM worldwide, according to Inter Press Service. First, HIV "is far more efficiently transmitted through the gut, hence leading to a far higher transmission probability in anal sex, for either a man or a woman -- around 18 times more likely than through vaginal transmission," the news service writes. Second, "because gay men can switch sexual roles in a way that is impossible among heterosexual couples -- acting as both the acquisition and transmission partner -- the efficiency of transmission among MSM networks appears to be far higher than previously understood," IPS adds, noting, "These two factors, the new research suggests, account for a full 98 percent of the difference between HIV epidemics among MSM and heterosexual populations."