Also called a yeast infection, thrush, and moniliasis, candidiasis is a very common cause of vaginitis. It is caused by a yeast-like fungus named Candida albicans. It is not often transmitted sexually, but still is considered an STD in the United States.
Virgins can also get it.
Candida albicans is normally present in healthy people in several areas in the body without causing any symptoms. Naming it as an STD has caused controversy in the medical community. It has been known to be transmitted sexually, but it requires a particular environment for growth, so it is not often sexually transmitted to the penis. It just doesn't grow well there. Pregnancy hormones can also cause candida when you are pregnant.