6 steps in treating breast infection Mastitis is an infection of the breast tissue that most commonly affects women who are breastfeeding. It often occurs within the first three months after delivery, but can happen any time during the breastfeeding period. Generally, lactation mastitis affects only one breast, not both. It results from a blocked milk duct, causing milk to remain in the breast that may lead to infection. A bacteria entering the breast through a break or crack