Recent evidence has linked bacterial vaginosis with acquisition of sexually transmitted diseases, including HIV infection. Prospective data show that women with bacterial vaginosis are at a significantly higher risk for HIV infection than those without bacterial vaginosis. The hallmark of bacterial vaginosis is the shift in the microbiology of the vaginal flora away from a lactobacilli-predominant milieu. In particular, lactobacilli that produce hydrogen peroxide are notably absent. Hydrogen peroxide-producing lactobacilli have been shown in vitro to be virucidal to HIV. Thus, the lack of these apparently protective bacteria, along with other local changes resulting from the shift in microflora, is thought to represent a biological risk factor for HIV acquisition.