The vagina is protected from infection by its acidic environment, which keeps the pH low. The lactobacillus acidophlus is the dominant bacteria found in a healthy vagina. The epithelium of the vagina is responsive to oestrogen, which breaks down glycogen to form lactic acid. This lactic acid keeps the pH lower than 4,5. The reduction of oestrogen during the uterine phase of menstruation, lactation, and menopause may result in the increase of the vaginal pH, making it susceptible to infection.