Bacterial vaginosis is caused by an imbalance of the organisms (flora) that naturally exist in the vagina. Normally, about 95% of vaginal flora are lactobacillus bacteria. (These types are unique to humans, different from the lactobacillus in yogurt.) These lactobacilli help keep the vaginal pH level low and prevent overgrowth of other types of organisms. Women with bacterial vaginosis have fewer lactobacillus organisms than normal and more of other types of bacteria. Experts do not yet understand what causes this imbalance.
Although the specific cause isn't known, a number of health and lifestyle factors have been linked to bacterial vaginosis. For more information, see What Increases Your Risk.