During breast-feeding, a yeast infection (called thrush when it occurs in the mouth) can be passed back and forth between the mother's breasts and the infant's mouth. The nipple of an infected breast becomes red or very pink and sore. If the infection extends into the milk ducts, it may cause pain throughout the breast and a sharp, burning pain in the breast while breast-feeding or between feedings.

An infant can be infected with yeast during delivery if the mother has a vaginal yeast infection, or the infection may develop later. The infant may then pass the infection to the mother during breast-feeding. Yeast infections of the breast may also occur when the mother is taking or has recently taken antibiotics. Cuts, scrapes, and other injuries to the nipple increase the risk of a yeast infection.

Thrush appears as white patches in the infant's mouth and a rash in the diaper area (one type of diaper rash).

Both the mother and infant must be treated for a yeast infection to prevent reinfection. For more information, see the topic Thrush.