Thrush is a type of mouth infection caused by an overgrowth of a fungus called Candida, more commonly known as yeast. It is also known as oropharyngeal candidiasis (OPC), oral candidiasis and moniliasis. Thrush can affect the corners of the mouth (cheilitis), insides of cheeks, tongue, palate and/or throat.

There are several types of Candida. However, a variety called Candida albicans causes the majority of thrush infections. Small amounts of these single–celled fungi are always present in the mouth, skin and digestive tract. In most cases, normal bacteria (flora) in the body prevent these fungi from multiplying.

However, certain factors, such as illness, medications or high glucose (blood sugar) levels can alter the environment inside the mouth or throat, causing Candida albicans to multiply rapidly and invade surrounding tissues. An overabundance of this fungus causes symptoms of thrush.

Thrush can affect anyone, but it occurs most often and strikes with greatest severity in people who have compromised immune systems, such as patients with HIV/AIDS or cancer, and those with uncontrolled diabetes. Thrush can also affect babies and toddlers and is extremely common in infants under the age of 6 months. Thrush can be transmitted during a vaginal delivery if the mother has a yeast infection. Symptoms may appear within seven to 10 days after birth. Older children and adults may develop thrush due to various conditions or medications that can promote overgrowth of the Candida fungus.  

Usually, thrush is not passed from person to person through casual contact. It may be transmitted between child and mother through breastfeeding.

Left untreated, symptoms of thrush may become persistent and painful. It can spread to the esophagus (esophagitis), respiratory tracts, lungs and gastrointestinal tract. In rare cases, invasive candidiasis (a condition characterized by infections in the organs and bloodstream) may occur, although this is more common with skin yeast infections. When it does occur, invasive candidiasis may affect the:

• Heart (endocarditis)
• Brain (meningitis)
• Joints (arthritis)
• Eyes (endophthalmitis)