Mastitis due to infection with Mycoplasma bovis is a fairly common occurrence in large, expanding dairy herds. While mycoplasma infections eventually show up in almost all herds, including "closed" herds, it is more commonly seen in herds containing cattle purchased from unknown sources. Mycoplasma mastitis can be extremely contagious and has earned a reputation for causing major outbreaks of untreatable mastitis. Whether or not a single mycoplasma infection develops into a major herd outbreak depends largely on the milking and hospital procedures used by the dairy.