Bacterial vaginosis, trichomoniasis, and vulvovaginal candidiasis are the most common infectious causes of vaginitis. Bacterial vaginosis occurs when the normal lactobacilli of the vagina are replaced by mostly anaerobic bacteria. Diagnosis is commonly made using the Amsel criteria, which include vaginal pH greater than 4.5, positive whiff test, milky discharge, and the presence of clue cells on microscopic examination of vaginal fluid. Oral and topical clindamycin and metronidazole are equally effective at eradicating bacterial vaginosis. Symptoms and signs of trichomoniasis are not specific; diagnosis by microscopy is more reliable. Features of trichomoniasis are trichomonads seen microscopically in saline, more leukocytes than epithelial cells, positive whiff test, and vaginal pH greater than 5.4. Any nitroimidazole drug (e.g., metronidazole) given orally as a single dose or over a longer period resolves 90 percent of trichomoniasis cases. Sex partners should be treated simultaneously. Most patients with vulvovaginal candidiasis are diagnosed by the presence of vulvar inflammation plus vaginal discharge or with microscopic examination of vaginal secretions in 10 percent potassium hydroxide solution. Vaginal pH is usually normal (4.0 to 4.5). Vulvovaginal candidiasis should be treated with one of many topical or oral antifungals, which appear to be equally effective. Rapid point-of-care tests are available to aid in accurate diagnosis of infectious vaginitis. Atrophic vaginitis, a form of vaginitis caused by estrogen deficiency, produces symptoms of vaginal dryness, itching, irritation, discharge, and dyspareunia. Both systemic and topical estrogen treatments are effective. Allergic and irritant contact forms of vaginitis can also occur.