"With regard to genetics, there are now 169 genes that are linked with inflammatory bowel disease. This means that the inflammatory bowel diseases – Crohn’s disease and ulcerative colitis – are complex genetic disorders. Related to the fecal microbiome, we now have an understanding that the bacteria in the colons of patients with IBD are different from healthy people without IBD. Our working definition of the cause of IBD is now that a patient with genetic susceptibility develops an inflammatory response to an altered fecal microbiome. Also, there is likely an important contribution of various environmental triggering events, such as cigarette smoking as a trigger for Crohn’s disease."
Understanding the Intestinal System - U-T San Diego