Streptococcal infections of the breast. Such an infection is usually suffered by lactating mothers and is medically known as Mastitis. Soreness and injury to the nipple can result in cracks in the skin or a blocked milk duct. This condition causes a build-up of milk within the breast. If the injury to the nipple allows for the entry of bacteria into the breast (these bacteria usually originate in the oral cavity of the child, or on the surface of the mother’s skin), an infection could arise within the breast.

Such infections are usually caused by Streptococcus agalactia or Streptococcus aureus. A localized swelling in the breast, or a lump, is usually the first manifestation of the infection. Alternatively, the entire breast may swell until it is abnormally large. There is also usually pus discharging from the nipple itself.