Yeast infections are a common cause of irritation of the vagina and vulva (area around the opening to the vagina). About 75 percent of women have a vaginal yeast infection during their lives. A kind of fungus called Candida causes vaginal yeast infections. It is normal to have some yeast in your vagina. Usually yeast is in balance with other organisms. But sometimes the balance is lost, and yeast overgrowth occurs. Hormonal changes can affect the acidity of the vagina and lead to yeast overgrowth. Another common cause of yeast infections is taking antibiotics.