Yeast infection is a term used more commonly to refer to fungal infections such as thrush or candidiasis. A yeast infection is caused by the overgrowth of the yeast called Candida, or the yeast family. Yeast infections can affect either the mouth or tongue called oral candidiasis, or the female genitals where the infection is referred to as vaginal yeast. In addition, the infection also occurs in babies, where it causes diaper rash.