mastitis (măstĪˈtĭs) [key], inflammation of the breast. Mastitis most commonly occurs in nursing mothers between the first and third weeks after childbirth, usually of the first child. It is an infection that results when bacteria enter through cracked nipples; the organisms may already be in the body and attack breast tissue weakened by injury. The breast becomes swollen and painful and a high fever may be present. Mastitis is usually easily treated with antibiotics. In severe cases, when an abscess forms within the breast, the suckling baby must be weaned completely. Chronic cystic mastitis is a common, noninfectious but often painful condition in women between 30 and 50 years old, in which cystic nodules develop in the breasts, giving them a lumpy appearance. It sometimes results from a hormonal imbalance. Biopsy may be necessary to distinguish the condition from breast cancer. Another type of mastitis may occur during puberty, and another is associated with other infectious diseases, e.g., mumps and tuberculosis.