Oral Yeast Infection (Thrush) Oral yeast, commonly called oral thrush or oral candidiasis, is a fungal infection in the mouth. It happens when a type of yeast called Candida grows more than usual. Candida can normally live in the mouth, throat, skin, and digestive tract without causing problems, but certain conditions can allow it to overgrow. Oral thrush can affect the tongue, inner cheeks, gums, roof of the mouth, tonsils, or throat. It is more common in infants, older adul