Thrush is caused by the overgrowth of a yeast called Candida albicans. Yeast is a type of fungus that can be found in a person’s intestines, mouth or skin. Fungi typically are normal and helpful to people. They stimulate the immune system, synthesize essential vitamins and protect the body against viruses and bacteria that cause illnesses. The body normally regulates the amount of yeast it has.

However, in some cases yeast can grow uncontrolled and cause thrush. This can occur for a number of reasons. For example, people with compromised immune systems lose the ability to fight off potentially harmful organisms, putting them at higher risk for thrush.

Diseases and therapies that may make a person more vulnerable to thrush infection include:

• Cancer. Both cancer itself and cancer treatments such as chemotherapy and radiation weaken the immune system, increasing the risk of thrush infection.
  
  
• Certain medications. Thrush may also develop in people who take certain types of medications, including chemotherapy drugs, antibiotics, corticosteroids, birth control pills and drugs to suppress the immune system.
  
  
• Diabetes mellitus. Poorly controlled diabetes can cause sugar levels in the saliva to increase, fostering the growth of thrush.
  
  
• Problems with the immune system. This includes the underdeveloped immunity in infants or declining immunity in the elderly. Chronic diseases can also take their toll on the immune system.
  
  
• Dry mouth (xerostomia). Occurs when the saliva glands fail to produce adequate moisture in the mouth, which disrupts the balance of microorganisms in a person’s mouth. This creates conditions that may be conducive to thrush. Patients who have had radiation treatment to the head or patients with  Sjögren’s syndrome, an autoimmune condition characterized by dry mouth, may be more likely to develop thrush. Certain drugs can also cause dry mouth.
  
  
• Vitamin deficiencies and/or malnutrition. Patients who do not get adequate amounts of vitamins may also have an increased risk of developing thrush.
  
  
• HIV/AIDS. The human immunodeficiency virus (HIV) damages the cells of the immune system, which makes the body more susceptible to infections such as thrush. Thrush rarely occurs in the early stages of AIDS, and usually only occurs when helper T cell counts drop below 350.
  
  
• Vaginal yeast infections. The same fungus that causes thrush also causes yeast infections. Newborn babies sometimes develop thrush after birth when their mothers have vaginal yeast infections at the time of delivery.
  
  
• Certain oral practices. Poorly fitting dentures and the overuse of antiseptic mouthwashes can also increase a patient's risk of developing thrush.

Thrush is commonly found in infants during the first few months of life. Young children have less fully developed immune systems than adults, so their bodies are less able to control the normal balance of yeast and bacteria. In addition, a baby with thrush can infect the mother’s nipple during breastfeeding.

Pregnancy itself can also cause thrush because the higher levels of estrogen and progesterone alter the balance of bacteria in the woman’s mouth and vagina. Additional factors that can increase the risk of thrush in a nursing mother or her child include a mother’s high sugar diet or low resistance to disease due to fatigue or other health issues.