Oral thrush. Lips of a 79 year old man with oral candidiasis (thrush, white patches). Candidiasis is an infection caused by the single-celled fungus Candida albicans. The fungus is commonly found on the skin and the mucous membranes of the mouth, digestive and respiratory tracts and the vagina, where it is usually harmless. However, the use of antibiotics or a suppressed immune system can lead to infection. Treatment is with antifungal drugs.