Johne's disease, also referred to as paratuberculosis, is a contagious disease that affects ruminant species worldwide, particularly dairy cows. It is caused by infection with Mycobacterium avium subsp. paratuberculosis (MAP). Most animals are infected the first couple months of life, however don't develop clinical signs until they are adults.

Transmission
Infected cattle shed the bacterium in their feces, colostrum, and milk. Young, more susceptible calves become infected through ingestion of contaminated feed, water, pasture grass, colostrum, or milk.