Mastitis is inflammation of the breast tissue. It is usually due to bacteria entering the nipple during breast-feeding, but it may also be caused by changes in levels of sex hormones—for example, the changes that occur at the start of puberty.
Typical symptoms include breast pain, tenderness, and swelling in one or both breasts. Bacterial mastitis during breast- feeding may also cause redness and engorgement, and may result in a breast abscess (collection of pus).
Mastitis due to infection may be treated with medication. Mastitis caused by hormonal changes usually clears up in a few weeks without treatment