- Vaginal yeast infections are caused by a fungus called Candida. Candida can live harmlessly in the vagina, but most women will have symptoms from a vaginal yeast infection at some point during their life. Antibiotics increase the risk for yeast infections, but it is unclear why. They may disrupt the balance of healthy bacteria in the vagina. This could make it harder for the body to fight off yeast infections. Researchers will give healthy women a common antibiotic or a placebo. They will study how the antibiotic affects bacteria and yeast in the vagina and other parts of the body. This will let researchers study the normal changes of healthy bacteria and yeast over time.