What does the epidemiologist concern himself with when studying an outbreak of Johne's disease? When the disease is first diagnosed in a herd many facts must be determined. First, how many animals are infected? This is difficult to determine as will be seen in the section on diagnosis. Second, where did the disease come from and what caused the breakdown? Since the natural incubation period has been determined (Hole, 1958), the investigator cannot look at past records of the herd and determine the source of the disease.