Thrush is a fungal infection caused by an overgrowth of a yeast called candida albicans. This yeast fungus lives on the skin, in the mouth, gut and vagina. It likes warmth and moisture. If your nipples become cracked or damaged, the candida fungus can get into your nipple or breast, causing a thrush infection.
If you are breastfeeding and experiencing pain in your nipples and breasts, this may be caused by a thrush (candida) infection in the breast.
It can make breastfeeding very painful, but it's easily treated.