Genital herpes (HSV-2) is the most common cause of genital sores worldwide, and the presence of genital sores is a significant risk factor for becoming infected with HIV. This study will test the effectiveness of twice-daily dosing of acyclovir, a commonly prescribed anti-herpes drug, in preventing HIV infection in HSV-2 infected women who sleep with men (WSM) and men who sleep with men (MSM).