Inflammatory bowel disease (IBD) is a group of conditions that cause inflammation in the colon and small intestine. The two principal types are Crohn's disease and ulcerative colitis. Specific bacteria have been found to work with the gut microbiome to cause IBD. Crohn's disease can affect other parts of the digestive system besides the colon and ileum, and causes ulcers and fistulas. Recent evidence suggests certain bacteria may play a role in triggering Crohn's disease.