Mastitis is caused by different bacteria. For one part, bacteria can enter through skin wounds along lymph and blood streams. For the other, the milk ducts can be infected at first with the inflammation spreading to the adjacent tissue in a second phase. Plugged ducts contribute to the proliferation of bacteria and precede mastitis in many cases. The transition from plugged ducts to mastitis is flowing. In about 90 % of the cases, mastitis is caused by the staphylococcus aureus bacteria. In the remaining cases, streptococcus and other germs cause the infection.