Clostridium difficile
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.