Ulcerative colitis (UC) is a type of inflammatory bowel disease (IBD) that may be caused by a person's immune system responding in an unusual way to bacteria normally found in the gut. Studies have shown that the gut bacteria in people with ulcerative colitis (UC) are different from the gut bacteria in people without ulcerative colitis (UC). Often, people with ulcerative colitis (UC) have fewer types of bacteria in their gut which can change the way that person's immune system works.