Mastitis is a very common breast disease, and the main patients are postpartum breastfeeding women. It is usually caused by an infection. It is also one of the major causes of maternal fever. So what is the cause of having mastitis? Small nipples (20%): Small or invaginated nipples that prevent breastfeeding, failure of pregnant women to correct invaginated nipples in time before delivery, and difficulty for babies to suckle. Excessive milk (20%): Excessive milk, incomplete emptying, the mother did not empty the excess milk from the breast in time. Milk duct inaccessibility (15%): Milk duct inaccessibility, inflammation of the milk duct itself, tumors and external pressure, fibers from the bra can also block the milk duct. Difficulty in suckling (25%): Difficulty in suckling can lead to breakage around the nipple, which is a major route of infection caused by bacteria invading along the lymphatic vessels. Infants often sleep with their nipples in their mouths (20%): If the child has oral inflammation, germs can directly invade the milk ducts and spread to the interstitial mammary glands, causing septic infection.