What is mastitis? Mastitis is an infection and inflammation of the breast, usually the fatty tissue of the breast, that causes redness, pain and swelling. As this swelling pushes on the milk ducts, it causes pain. Mastitis is usually caused by an infection with the bacteria Staphylococcus aureus. These bacteria are normally present on your skin, but cause problems when they enter the body. Bacteria causing mastitis enter through a break or crack in the skin of the breast, usually on the nipple. In fact, mastitis usually occurs in women who are breastfeeding because the nipples often become dry and irritated and can become cracked during nursing. This allows the bacteria to invade breast tissue, in particular the milk ducts and milk glands. Mastitis in a nonbreastfeeding woman is more common after menopause than before. In very rare cases, this may indicate the presence of another primary disease, such as breast cancer. At the same time, a clogged milk duct can mimic mastitis. While a woman adjusts to breastfeeding a new infant, the milk ducts inside the breast can become clogged, causing tenderness, redness, lumps and even heat under the skin surface, but without infection. It is possible in many cases to determine yourself if you have a clogged milk duct as opposed to mastitis. You can usually relieve a clogged milk duct by massaging the area. If these symptoms persist, however, or if you develop fever and muscle pains or body aches, it is more likely that you have indeed developed mastitis. Fortunately, mastitis can be easily treated. While mastitis is almost never an emergency, left untreated it can lead to a breast abscess, which is a collection of pus in a hollow area in the breast. Your doctor may need to drain the abscess. A wiser course is to never let mastitis lead to an abscess. Seek prompt medical care if you find you have trouble relieving breast engorgement even while nursing, if you develop heat, tenderness, lumping or swelling in the breast—whether breastfeeding or not—or if you develop flu-like symptoms. Seek prompt medical care if you are breastfeeding and the symptoms do not resolve by massaging the area to unclog a milk duct.