Causes

Milk that is trapped in the breast is the main cause of mastitis. Other causes include:

• A blocked milk duct. If a breast doesn't completely empty at feedings, one of your milk ducts can become clogged. The blockage causes milk to back up, leading to breast infection.
• Bacteria entering your breast. Bacteria from your skin's surface and baby's mouth can enter the milk ducts through a crack in the skin of your nipple or through a milk duct opening. Stagnant milk in a breast that isn't emptied provides a breeding ground for the bacteria.