Oral thrush occurs when a yeast infection develops on the inside of your mouth and on your tongue. This condition is also known as oral candidiasis, oropharyngeal candidiasis, or, simply, thrush. The Candida albicans (C. albicans) fungus causes oral thrush. A small amount of this fungus normally lives in your mouth without causing harm. However, when the fungus begins to grow uncontrollably, an infection can develop in your mouth. Oral thrush most often occurs in infants and toddlers. It causes white bumps to form on the inner cheeks and tongue. These growths usually go away once treatment is received. Oral thrush is typically mild and rarely causes complications. However, the condition can be problematic for those with weakened immune systems.