A yeast infection (candidiasis) is caused by the fungus Candida albicans, found normally in small amounts in the vagina, in the mouth and digestive tract, and on the skin. It usually causes no problems because other bacteria in the vagina keep it in check. However, an impaired immune system may lead to a growth in the fungus that causes yeast infections.
Taking antibiotics for other infections may cause a yeast infection by killing the bacteria that protect against fungal infection and allowing Candida albicans to grow. Women with uncontrolled diabetes or other conditions that suppress the immune system are at higher risk for candidiasis. Pregnancy and obesity are other conditions favorable to allowing the fungus to grow.
Having intercourse with someone who has a yeast infection, may cause sexually transmitted fungal infections.