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. However, when conditions are present that allow the yeast to grow uncontrolled, it 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. These include Candida tropicalis, Candida krusei, Candida parapsilosis, and Candida glabrata.¹

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 factors 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. Thrush is a common condition in infants during the first few weeks of life.
  • Older adults are more likely to develop thrush because they may have weaker immune systems.
  • People with human immunodeficiency virus (HIV) infection have a high likelihood of developing thrush. About 90% of people with HIV develop thrush at some point during the course of their disease.²
  • People with diabetes are more prone to thrush because high blood sugar levels promote overgrowth of the candida fungus.
• 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 altering 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 corticosteroids medications may lead to thrush. These medicines may upset the balance of bacteria in the mouth.

How thrush spreads

The yeast causing 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. Symptoms of thrush generally appear 3 to 7 days after birth. Newborns and infants have an immature immune system and have not fully developed a healthy balance of bacteria and fungus 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. However, a child with a weakened immune system may develop thrush by sharing infected toys or pacifiers with a child who has the infection. A child with thrush spreads the thrush fungus onto anything the child puts in his or her mouth. Another child may then get thrush by putting an infected object into his or her mouth.
• Adults who wear false teeth (dentures) are at a higher risk for developing thrush and spreading it to others. This may happen when another person touches something with the yeast on it and then touches his or her mouth.