Yeast are part of the normal flora in the mouth. They ordinarily do not cause symptoms. Certain conditions, such as antibiotic use, can disturb the natural balance of microorganisms in the mouth and allow the overgrowth of Candida to cause thrush.
Overgrowth of Candida albicans can also occur in the gastrointestinal tract, skin, and other areas of the body. Vaginal yeast infections, some forms of diaper rash and other skin rashes that emerge in moist, warm areas of skin are all forms of yeast infection.
Thrush is usually a minor and easily addressed problem, but it can be more serious for those with immune system disorders, such as AIDS. Treatment is with antifungal medications.