The mouth normally contains many microorganisms. One of these normal microorganisms is the fungus Candida albicans, which is the same fungus associated with vaginal yeast infections or other Candida infections. The growth of Candida is normally kept under control by the presence of normal bacteria.

Uncontrolled overgrowth of Candida in the mouth may be caused by factors that reduce the natural resistance, such as illness, stress, long-term use of corticosteroids or medications that suppress the immune system, and immune disorders such as (AIDS).

It may also be caused by conditions that upset the balance of normal microorganisms in the mouth. This is most commonly associated with use of antibiotics, but it may also occur with uncontrolled diabetes mellitus and with the hormonal changes associated with pregnancy or the use of birth control pills.

Oral thrush is most common in infants and toddlers, in the elderly, and in people who are debilitated or whose immune system has been suppressed by disease or medical treatments.

Thrush in infants is common and is usually not associated with other diseases. Infant thrush that resists treatment or recurs frequently should raise suspicions for an underlying disorder.

In adolescents, young adults, and middle age adults, an oral Candida infection should always be viewed as a possible symptom of an underlying medical problem, such as diabetes or HIV infection, and should be evaluated.