An inflammation or infection of the breast. It occurs in about 1% of new mothers and is more common in women who are breast-feeding. The most common bacteria cause is Staphylococcus aureus. It is believed the bacteria get into the breast through cracks in the skin. This disorder is more likely to occur if the breast feeding mother has skin irritation in the area of the nipple. In some cases mastitis can lead to the development of a breast abscess (fluid collection of infectious material).