Candida , the yeast that causes thrush, is normally present in small amounts in the mouth and on other mucous membranes. It usually causes no harm. But when conditions are present that let the yeast grow uncontrolled, the yeast invades surrounding tissues and becomes an infection.
Thrush is most commonly caused by the yeast Candida albicans. Less frequently, other forms of Candida can lead to thrush.
There are many types of bacteria in your mouth that normally control the growth of Candida. Sometimes a new type of bacteria gets into your mouth and disrupts the balance of the organisms already there, allowing Candida to overgrow. Health conditions and other things may also be involved.