Mastitis is an inflammation of the breast that occurs most commonly in breast-feeding women; it can be related to tissue injury, infection, or both. Breast-feeding mastitis is typically caused by bacteria that enter the breast through a cracked or sore nipple.
Signs of infection may include:
Treatment usually involves antibiotic medicines, keeping up milk flow, and measures to relieve discomfort. While they are being treated, most women can safely continue to breast-feed or pump breast milk.
WebMD Medical Reference from Healthwise