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.
- An impaired ability to fight infection (weakened immune system) increases your risk for thrush. A normal immune system is usually strong enough to prevent Candida from overgrowing.
- Infants are more prone to thrush because an infant's immune system is not fully developed.
- Older adults are more likely to develop thrush because they may have weaker immune systems.
- People with human immunodeficiency virus (HIV) infection are very likely to get thrush.
- People with diabetes are more prone to thrush because high blood sugar levels promote overgrowth of the Candida yeast.
- Having a dry mouth (xerostomia) can lead to thrush. Dry mouth can result from overuse of mouthwashes or from certain conditions such as Sjögren's syndrome.
- Pregnancy increases your risk for thrush. Hormone changes during pregnancy can lead to thrush by changing the balance of bacteria in the mouth.
- Not caring for your teeth can make thrush more likely to develop. This is especially true in people who have false teeth (dentures).
- Taking antibiotic or inhaled corticosteroid medicines may lead to thrush. These medicines may upset the balance of bacteria in the mouth.
How thrush spreads
The yeast that causes thrush can pass from one person to another in different ways.
- A newborn can get thrush during birth, especially if his or her mother had a vaginal yeast infection during labor and delivery. Newborns and infants have an immature immune system and have not fully developed a healthy balance of bacteria and yeast in their mouths. Because of this, thrush is common during the first few months of life.
- In otherwise healthy toddlers and older children, thrush is usually not contagious. But a child with a weakened immune system may get thrush by sharing infected toys or pacifiers with a child who has the infection. A child who has thrush spreads the thrush yeast onto anything the child puts in his or her mouth. Another child may then get thrush by putting a contaminated object into his or her mouth.
- Adults who wear false teeth (dentures) are at a higher risk for getting thrush and spreading it to others. A person can get thrush by spreading the yeast from their hands to their dentures. And a person with dentures may spread the yeast by handling their dentures and then contaminating an object that another person touches or puts into his or her mouth.