Mastitis is an infection of the tissue of the breast that occurs most frequently during the time of breastfeeding. This infection causes pain, swelling, redness, and increased temperature of the breast. It can occur when bacteria, often from the baby's mouth, enter a milk duct through a crack in the nipple. This causes an infection and painful inflammation of the breast.

Breast infections most commonly occur one to three months after the delivery of a baby, but they can occur in women who have not recently delivered as well as in women after menopause. Other causes of infection include chronic mastitis and a rare form of cancer called inflammatory carcinoma.

• 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. Under the areola are lactiferous ducts. These fill with milk during lactation after a woman has a baby. When a girl reaches puberty, changing hormones cause the ducts to grow and cause fat deposits in the breast tissue to increase. The glands that produce milk (mammary glands) that are connected to the surface of the breast by the lactiferous ducts may extend to the armpit area (axilla).

• A breast infection that leads to an abscess (a localized pocket or collection of pus) is a more serious type of infection. If mastitis is left untreated, an abscess can develop in the breast tissue. This type of infection may require surgical drainage.