One study found ileocecal Crohn's disease is associated with viral species from the enterovirus genus (but note that all the study cohort with ileocecal Crohn's disease had disease-associated mutations in either their NOD2 or ATG16L1 genes). Crohn's disease is associated with Mycobacterium avium subspecies paratuberculosis. In a murine model, Crohn's disease is precipitated by the norovirus CR6 strain, but only in combination with a variant of the Crohns susceptibility gene ATG16L1, and chemical toxic damage to the gut. In other words, in this mouse model, Crohns is precipitated only when these three causal factors (virus, gene, and toxin) act in combination.
