Mastitis is a difficult problem to comprehend because it is a disease caused by many factors. Microorganisms are responsible for the infection, but for them to enter the mammary glands and establish themselves to the point that they cause an infection, a multitude of factors may be involved. There are many such factors (e.g. hygiene, housing, climate, milking machines, feed, genetics) acting simultaneously. It is even more difficult to generalize about the relative importance of each one, as certain factors affect certain microorganisms in particular.
Klastrup and his coworkers21 estimate that 25% of the susceptibility to infection is attributable to environmental factors, 20% to genetic factors, and 50% to herd management.