The notion of a protective vaginal flora is relatively new. Resident flora manifest colonization resistance to prevent or reduce the likelihood of exogenous microorganisms, viruses, bacteria, yeast, or parasites becoming established in the lower genital tract of women following sexual (HIV, Neisseria gonorrhoeae, Escherichia coli, Candida albicans, Trichomonas vaginalis) or nonsexual (uropathogenic E. coli) transmission. The concept of preserving or reestablishing protective flora has been hastened by several factors, including the potential widespread use of vaginal microbicides, the increased heterosexual spread of HIV, and the imminent availability of exogenous Lactobacillus species probiotic therapy.