Johne's disease is spread in cattle primarily through the causative organism (a bacterium called Mycobacterium paratuberculosis) being shed in the feces of an infected animal and ingested by other animals. As infected animals age and the disease progresses, more and more bacteria are shed in the feces each day.
Animals usually become infected very early in life. The older the animal, the more resistant it is to infection from this bacteria. Calves can be born with the infection if their dam had the disease.
