Mastitis is an infection that often affects women who are breastfeeding or those who have had a break or crack in the skin. The dry and cracked skin around the nipple can allow bacteria from the skin surface to enter the breast duct. Once in the duct, the bacteria can grow and cause redness and inflammation. The inflamed cells release substances that fight the infection, but those substances can also cause swelling and increased blood flow in the affected tissue. These changes often cause the surrounding area to be painful and the skin to become red and warm to the touch.

Mastitis is usually treated with antibiotics. If antibiotic treatment does not help, and inflammatory breast cancer has not been ruled-out, then a biopsy of the skin may be needed to confirm that it is not cancer.

Some cases of mastitis lead to a breast abscess or collection of pus made up of inflammatory cells and fluid. These abscesses are usually treated by draining the pus.