A vaginal yeast infection is caused by a yeast fungus. The fungus is called Candida albicans. While yeast is common in the vagina, it can cause problems when it grows excessively. This excess growth causes the uncomfortable symptoms.

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.