Prescription drugs now used to treat human immunodeficiency virus infection in adults may prevent the vaginal transmission of HIV, researchers have found. Using a highly sophisticated human/mouse chimera or “humanized mouse” model, the researchers discovered that anti-retroviral drugs given daily before and after exposure to HIV can prevent vaginal transmission of the virus that causes AIDS. Worldwide, the vast majority of newly acquired HIV infections occur through unprotected vaginal sex with an infected partner.