Mastitis is an inflammatory reaction of the body to an infection of the secretory part of the mammary gland. The causes of mastitis are divided by Gram staining into gram-positive and gram-negative, and according to the etiology into contagious and environmental causes. Contagious pathogens are most transmitted during milking, and cause long-lasting, chronic mastitis of mostly subclinical course. In contrast, environmental pathogens typically cause short-term infections, but with more frequent clinical manifestations. Infected cows are detected by the diagnosis of mastitis, which is most often carried out by indirect or direct methods of determining the number of somatic cells and bacteriological examination of milk. The Zagreb mastitis test is a quick and simple orientation method for estimating the number of somatic cells that is performed in field conditions. Somatic cells are made up of epithelial cells and leukocytes. They are normally found in milk, but their number and ratio change depending on inflammatory changes in the mammary gland. The movement of the number of somatic cells mainly depends on the occurrence of mastitis, but also on the stage of lactation, the age of the animal, the season and other diseases of the dairy cow. Milk with an increased number of somatic cells has an altered quality compared to the milk of a healthy cow. Milk contains a lower proportion of milk fat and lactose, and an increased proportion of protein. Furthermore, electrical conductivity is increased due to elevated concentrations of chlorine and sodium in milk. It also has reduced thermal stability and reduced ability to curdle and form lumps. Such milk has a rancid taste and is not suitable for the production of condensed milk, milk powder and sterilized milk. The shelf life of cheeses produced from such milk is reduced due to the retention of a larger amount of water. Proper and regular disinfection of udder teats before and after milking raises the yield and quality of milk and reduces the cost of mastitis treatment and increases the income of farm.