Mastitis is an inflammation of the breast tissue or breast duct. Lactational mastitis is the most common form and occurs in 10%-30% of lactating people. It is caused by an infection of stagnant milk in blocked ducts. The majority of cases occur in the first 4 weeks postpartum or at the end of breastfeeding.

Risk factors include missed feedings, nipple damage from breastfeeding attempts, infant attachment difficulties, rapid weaning, pressure on the breasts from brassieres or other compressive clothing, history of prior mastitis, and incomplete breast emptying with feedings.

Infection is most commonly due to Staphylococcus aureus or Staphylococcus albus. Escherichia coli and some streptococcal species are rare but may also be encountered.

Patients will present with sudden onset of fever and myalgias as well as warmth, tenderness, swelling, and an erythematous plaque. Complications include recurrence and breast abscess formation.

Other, rarer forms include tuberculosis mastitis.