Thrush is a yeast infection of the mucus membrane lining of the mouth and tongue.

Thrush is caused by forms of a fungus called Candida. A small amount of this fungus lives in your mouth most of the time. It is usually kept in check by your immune system and other types of germs that also normally live in your mouth.

However, when your immune system is weaker, the fungus can grow, leading to sores (lesions) in your mouth and on your tongue. The following can increase your chances of getting thrush:

• Taking steroid medications
• Having an HIV infection or AIDS
• Receiving chemotherapy for cancer or drugs to suppress your immune system following an organ transplant
• Being very old or very young
• Being in poor health

Thrush is commonly seen in infants. It is not considered abnormal in infants unless it lasts longer than a couple of weeks.

Candida can also cause yeast infections in the vagina.

People who have diabetes and had high blood sugar levels are more likely to get thrush in the mouth (oral thrush), because the extra sugar in your saliva acts like food for Candida.

Taking high doses of antibiotics or taking antibiotics for a long time also increases the risk of oral thrush. Antibiotics kill some of the healthy bacteria that help keep Candida from growing too much.

People with poorly fitting dentures are also more likely to get thrush.