The causes of mastitis are firstly bacterial invasion and secondly the accumulation of milk. The accumulation of milk mainly occurs during breastfeeding, due to the position of holding the child or unintentional bumps by the child, which cause damage to the milk ducts, constriction of the ducts, leading to a large amount of milk being trapped inside the ducts. Since milk is an excellent culture medium, it results in bacterial growth. Bacterial invasion usually occurs through cracked or fissured nipples, allowing bacteria to enter the milk ducts and into the breast tissue, causing mastitis. Another situation is a decrease in the body's immune system, for example, due to poor sleep or emotional stress. These are the main causes of acute mastitis.