Mastitis is the most economically significant disease of dairy cattle. It is an inflammation of a female cow's mammary gland. It is most often caused by pathogenic bacterial infection. It occurs occasionally in lactating cows nursing calves, most commonly in the drying-off period (the end of milk production, when the glands are reducing and stopping the production of milk). Mastitis can occur in one or both breasts. Milk from inflamed mammary glands may appear normal or may be abnormal in color or consistency.