Most abscesses are caused by a bacterial infection. They are often linked to mastitis, an inflammation of your breast tissue, particularly the milk ducts and glands if you are breastfeeding.
Infections can occur when bacteria from the skin's surface or baby's mouth enter the milk ducts through a break or crack in the skin of the nipple, or through a milk duct opening. If the infection is not treated, an abscess can form.
Women who are not breastfeeding can also develop a breast abscess if bacteria enters the breast through a sore or cracked nipple, or a nipple piercing.