Thrush is a type of mouth infection caused by an overgrowth of a fungus called Candida, more commonly known as yeast. It may also be called oral thrush, oropharyngeal candidiasis or OPC. Newborns are among the groups at the highest risk for thrush. In most cases, thrush is easily treatable and is not viewed as a serious health threat.

There are several types of Candida. However, a variety called Candida albicans causes the majority of oral thrush infections. Small amounts of these single-celled fungi are always present in the mouth, skin and digestive tract. 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.

Oral thrush can affect anyone, but is extremely common in children under the age of six months. It also can affect older babies and toddlers. Usually, oral thrush is not passed from person to person through casual contact. However, it may be transmitted between child to mother through breastfeeding. If an infant who is breastfeeding has thrush, the mother should use antifungal cream on her nipples to prevent further spreading of the disease in the infant.

Thrush can also be transmitted when a child with a developing immune system shares items contaminated with the Candida fungus. Examples of such items include pacifiers or toys that have been in oral contact with an infected child. 

Thrush is also more prevalent in:

• People with uncontrolled diabetes mellitus
• Individuals with compromised immune systems, particularly those with HIV or AIDS

Left untreated, symptoms of oral thrush may become persistent and painful. It can spread to the esophagus (esophagitis), the skin or the vagina (yeast infection). In rare cases, invasive candidiasis (a condition characterized by infections in the organs and bloodstream) may occur, affecting the:

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