Antibiotics. Taking antibiotics is one of the most common causes of a vaginal yeast infection. Antibiotics and some other medications change the level of bacteria and other organisms in the vagina -- such as the bacteria lactobacillus acidophilus, which normally helps keep yeast cells under control. Medications can also change vagina's acidity, called the "pH balance," allowing yeast to overgrow.
Pregnancy. Pregnant women have high levels of estrogen, which can cause a yeast infection during pregnancy.
A weakened immune system. Having a condition that weakens the immune system, such as HIV or AIDS, can prevent the body from fighting off a yeast infection. Some women find that being overly stressed or having a cold or flu, which already taxes the immune system, can raise their risk of a yeast infection.
Douching upsets the natural balance of yeast and bacteria that live in the vagina and kills off some of the healthy bacteria that normally prevent yeast from overgrowing.
Having an occasional yeast infection is an uncomfortable, though rarely serious, problem. But see your doctor if you have frequent, repeat vaginal yeast infections, which may be an indication of another underlying infection.