Johne's disease or paratuberculosis is caused by the mycobactium species, Mycobacterium avium subspecies paratuberculosis. It affects the small and large intestine of ruminants. It causes thickening of the tissues which results in diarrhea and decreased absorption of nutrients. Mycobacterium avium subspecies paratuberculosis has been implicated by some research as the cause of Crohn's disease in humans.