The breast is composed of several glands and ducts that lead to the nipple and the surrounding colored area called the areola. The milk-carrying ducts extend from the nipple into the underlying breast tissue like the spokes of a wheel.

Mastitis is an infection of the tissue of the breast that occurs most frequently during the time of breastfeeding. It can occur when bacteria, often from the baby's mouth, enter a milk duct through a crack in the nipple.