A vaginal yeast infection is caused by a yeast fungus called Candida. While yeast is common in the vagina, it can cause irritating symptoms when it grows excessively.

Vagina
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Yeast grows in conditions that are less acidic. Vaginal fluids are most often mildly acidic, but this can change. For example, acid levels can decrease during menstrual flow. "Good" bacteria also help keep vaginal secretions acidic and keep yeast levels in check. Conditions that decrease the good bacteria will also increase the chance of a yeast infection.