"C. diff "can live in the human colon (the large intestine) without causing any problems. About 2-5% of adults have "C. diff" living in their colons. However, in some people, "C. diff "causes serious illness. In these people, "C. diff "bacteria grow out of control in the colon. The bacteria attack the lining of the intestines. This causes a problem called "C. diff "colitis. Colitis is inflammation (swelling) of the colon.