Mastitis develops when a pathogen that typically originates in the nursing infant’s nose or pharynx invades breast tissue through a fissured or cracked nipple and disrupts normal lactation. The most common pathogen of this type is Staphylococcus aureus; less commonly, it’s Staphylococcus epidermidis or beta-hemolytic streptococci. Rarely, mastitis may result from disseminated tuberculosis or the mumps virus. Predisposing factors include a fissure or abrasion on the nipple; blocked milk ducts; and an incomplete let-down reflex, usually due to emotional trauma. Blocked milk ducts can result from a tight bra or prolonged intervals between breast-feedings.

Causes of breast engorgement include venous and lymphatic stasis and alveolar milk accumulation.

Mastitis may develop anytime during lactation but usually begins 3 to 4 weeks postpartum with fever (101° F [38.3° C] or higher in those with acute mastitis), malaise, and flulike symptoms. The breasts (or, occasionally, one breast) become tender, hard, swollen, and warm. Unless mastitis is treated adequately, it may progress to breast abscess.