Men who have sex with men are the risk population most heavily impacted by HIV in the United States by any measure; in 2009, at least 61% of new HIV infections were estimated to have occurred in MSM. MSM are experiencing an increase in HIV transmission that has been occurring since at least 1990, and accelerated in multiple North American and European countries from 2000-2005. The expansion of the HIV epidemic has been proposed to be attributable, in part, to the extent to which the internet has facilitated sexual connectivity among MSM.