Most breast abscesses occur as a complication of mastitis, which is a bacterial infection that causes the breast to become red and inflamed.

Mastitis usually affects breastfeeding women, but it can also sometimes occur in women who aren't breastfeeding.

Women who smoke have an increased risk of developing non-breastfeeding mastitis – otherwise known as periductal mastitis.

Bacterial infection

Most abscesses are caused by bacterial infections. The most common bacteria that causes mastitis is Staphylococcus aureus.

The bacteria usually enter the breast through small cracks or breaks in the skin of the nipple, which can sometimes develop during breastfeeding.

Infections can also be caused by an overgrowth of bacteria that usually exist quite harmlessly within the milk ducts (the tiny tubes inside the breast that carry milk). An overgrowth of bacteria can occur if stagnant milk collects in a blocked milk duct.

When bacteria enter your body, your immune system (the body’s natural defence) tries to fight them off by sending white blood cells to the affected area. The white blood cells attack the bacteria, which causes some of the tissue at the site of the infection to die, creating a small, hollow pocket.

The pocket starts to fill with pus, forming an abscess. The pus contains a mixture of dead tissue, white blood cells and bacteria. As the infection progresses, the abscess may get bigger and more painful as more pus is produced.