Acute mastitis, frequently complicated by breast abscess formation, typically develops in the postpaum period, when the nipples are first subjected to the physical stresses of breast feeding. Bacteria enter the breast cracks in the nipple, and flourish in the microenvironment of the lactating breast. Acute mastitis causes redness, pain, and swelling in the affected breast; Staph. aureus is the most common pathogen. Although endocarditis could send septic emboli to the breast, Staph. aureus endocarditis is more commonly associated with bacteremia and fevers, proliferative glomerulonephritis, valvular dysfunction, and emboli to the brain, kidneys, hea, and gut. Inflammatory breast carcinoma does not invoke an inflammatory response, and it is not associated with an infection. Inflammatory breast carcinoma is characterized by tumor spread into the dermal lymphatics, producing diffuse induration and skin tenderness with the typical peau d'orange appearance. Mastitis arising in the perimenopausal period is usually a chronic mastitis, caused by obstruction of ducts due to inspissated secretions. Chronic mastitis is sterile. The breast tissue shows lymphocytes and plasma cells surrounding dilated ducts filled by cellular debris. Ref: Wyatt C., Butterwoh IV J.F., Moos P.J., Mackey D.C., Brown T.G. (2008). Chapter 17. Pathology of the Male and Female Reproductive Tract and Breast. In C. Wyatt, J.F. Butterwoh IV, P.J. Moos, D.C. Mackey, T.G. Brown (Eds),Pathology: The Big Picture.