Oral thrush is a yeast infection of the mouth caused by uncontrolled growth of an organism called Candida albicans.
Candida albicans is an organism that normally makes a quiet home for itself on your skin and doesn't bother anyone. We all carry this organism on our skin, in our mouth, in our gastrointestinal tract (gut) and, for women, in the vagina.
Occasionally the yeast multiplies uncontrollably, causing pain and inflammation. When this occurs, it is called candidiasis. It is also known as moniliasis or a yeast infection.