Trichomoniasis is an infection caused by Trichomonas vaginalis, a single-celled, anaerobic, protozoan parasite that is shaped like a turnip and has three or four anterior flagella. This parasite feeds on the vaginal mucosa and ingests bacteria and leukocytes. In sexually active adolescents, approximately 2% to 3% have T. vaginalis.

Vulvovaginal candidiasis is caused by Candidaalbicans (most often), C. glabrata, or C. tropicalis. These organisms are normally present in approximately 50% of women and cause no symptoms until the vaginal environment is altered. Contributing factors to altering the vaginal environment and causing an overgrowth of Candida are taking broad-spectrum antibiotics, which alter the protective bacterial flora; higher hormone levels from birth control pills and pregnancy, which increase glycogen stores that facilitate yeast growth; and diabetes mellitus or HIV infection, which alters the immune system. Repeated candida infections may be an indicator of unrecognized HIV infections.

Bacterial vaginosis (nonspecific vaginitis) is characterized by an imbalance in the vaginal flora (absence of the normal Lactobacillus species) and an overgrowth of Gardnerella,Mycoplasma species, and anaerobic bacteria. The anaerobe raises the vaginal pH, producing favorable conditions for bacterial growth. Cervicitis and urethritis are frequent manifestations of gonococcal or chlamydial infections and result from infection by Neisseria gonorrhoeae or Chlamydia trachomatis, but other agents may also cause vaginitis. Group B streptococcus is a cause of vaginitis, and patients culturing positive have vaginal mucosa with small, macular blotches of erythema. Group A streptococcal is rarely a concern in adults but is the causative factor in 21% of vulvovaginitis of prepubescent girls. Atrophic vaginitis may result from estrogen deficiency.