For decades, scientists have suspected Crohn's is caused by bacteria that cause a similar intestinal disorder in cattle, sheep and goats, but there was little evidence.
Until now, microbiologists have been unable to culture bacteria from the blood of patients with the disease.
Common symptoms of Crohn's disease include abdominal pain, diarrhea, weight loss and fever. Inflamed parts of the intestine may need to be surgically removed.
The bacteria, called Mycobacterium avium subspecies paratuberculosis or MAP, are a relative of the tuberculosis germ.