There are millions of cases of vaginal yeast infection in the U.S. every year. So what is it? A vaginal yeast infection causes itching and irritation of the vagina and vulva (folds of skin outside the vagina). Yeast is a type of fungus that normally live in the vagina, along with bacteria and other organisms. When something causes an imbalance in these organisms, yeast grow and cause irritation. A yeast infection is not an STD and clears up quickly with treatment.