A vaginal yeast infection is caused by an overgrowth of yeast organisms that normally live in small numbers in the vagina. Factors that encourage an excess growth of vaginal yeast include:

• Using antibiotics.
• Having high estrogen levels (hyperestrogenemia), such as during pregnancy, hormone replacement therapy (HRT or ERT) use, high-dose birth control pill use, and the follicular phase of the menstrual cycle.¹
• Having diabetes, especially if your blood sugars are not well controlled and tend to be high.
• Having an impaired immune system.
• Using corticosteroids, such as prednisone.
• Being overweight.
• Having the genital area exposed to moisture for a long period of time (for example, wearing a wet bathing suit all day) or living in a warm, humid climate.

Most yeast infections involve Candida albicans (C. albicans), a yeast that can be treated effectively with standard yeast infection medicine. Recurrent vaginal yeast infections tend to involve non–C. albicans types of yeast, which are more resistant to the standard treatments for C. albicans yeast infections.

Chronic yeast syndrome, a condition in which excess growth of yeast supposedly affects the whole body, has not been medically proved.