The Candida fungus, which is also called yeast, causes thrush in adults, according to WebMD. Although Candida yeast is present in small amounts in the mouth, skin and digestive tract of healthy people, it is kept in balance by other microorganisms and bacteria in the body. When this delicate balance is disturbed by stress, sickness or certain medications, the Candida fungus can thrive, resulting in thrush.
While thrush specifically refers to a Candida infection of the mouth, a Candida infection can occur in other areas of the body as well, according the WebMD. It is a common infection in children, people with weakened immune systems and older adults. Thrush produces small, white lesions on the tongue and cheeks, and, in some cases, on the roof of the mouth, gums or tonsils. It can also be associated with pain and difficulty swallowing, and if the infection spreads beyond the esophagus, it can cause fever.
Some medications that can upset the balance of Candida fungus in the body include antibiotics, corticosteroids and birth control pills, according to WebMD. Uncontrolled diabetes, HIV infection, cancer, dry mouth and the hormonal changes that occur with cancer can all increase an adult's risk of developing thrush.