Thrush is an infection of the breast by yeast (Candida albicans). This same yeast can cause vaginal yeast infections, infect a baby’s mouth and cause a diaper rash. C. albicans is found on and in everyone’s body. Sometimes the balance between the different normal flora of the body gets out of whack and yeast can take over, resulting in symptoms of a yeast infection. Information Sheet #461 Thrush & the Breastfeeding Family contains detailed information.