Inflammation of the mammary gland. This condition is most frequently caused by infection of the gland with bacteria that are pathogenic for this organ. It has been described in humans, cows, sheep, goats, pigs, horses, and rabbits. Mastitis causes lactating women to experience pain when nursing the child, it damages mammary tissue, and the formation of scar tissue in the breast may cause disfigurement. It also is the most economically important disease of dairy cows.