Thrush is a yeast infection that causes white patches in the mouth and on the tongue. Thrush is most common in babies and older adults, but it can occur at any age. Thrush in babies is usually not serious.You get thrush when a yeast called Candida, normally found on the body, grows out of control.eople who have certain health problems, such as diabetes or HIV, are also more likely to get thrush. The most common symptoms of thrush are white patches that stick to the inside of the mouth and tongue. In babies, it is easy to mistake thrush for milk or formula. It looks like cottage cheese or milk curds.