Yeast infections may occur due to any number of reasons. New yeast entering the vaginal area will lead to a yeast infection. Yeast infections can also be caused, however, when there is more yeast in the vagina than bacteria. That can be caused by using antibiotics for another condition. In the case of the inner vagina being hurt (possible side effect of chemotherapy), a woman may become prone to yeast infections too. Women with a suppressed immune system, diabetes mellitus, or women who are pregnant or women taking oral contraceptives can be more apt to get yeast infections then do other women.
Besides not being an STD, yeast infection can also hit women who are not even sexually active. Although, once in awhile when a woman suffering from yeast infection has intercourse with a man, his penis may get yeast infection with skin discomfort.