What causes thrush (oral candidiasis)?

Thrush is caused by a yeast infection. Yeast is a form of fungus that lives throughout the environment, and in your body. Your mouth naturally contains a small amount of yeast called Candida. Normally, your immune system keeps Candida under control. However, if your immune system has been weakened for any reason (such as taking certain medications or simply being in poor health), Candida may flourish and cause the symptoms of thrush, which include white sores in the mouth and on the tongue. These sores may bleed.

Oral thrush is primarily a fungal infection caused by a strain of yeast called Candida Albicans. In a healthy person, this yeast is present in only tiny amounts. Many different factors can contribute to an overgrowth of Candida resulting in a case of thrush. These factors include dry mouth, poor oral home care, or using an oral appliance (like a denture or a retainer) that hasn't been kept clean. Taking antibiotics will wipe out bacteria in the mouth, allowing Candida to proliferate. Anything that decreases your immune system increases your risk of oral thrush. This includes medications like corticosteroids, chronic illnesses like diabetes, stress, certain anemias, radiation, and chemotherapy. Infants, with their developing immune systems, are prone to oral thrush. Also, individuals with HIV are at high risk for developing oral thrush.