The bacterium Clostridium difficile or C. difficile is a major cause of colitis and diarrhea that occurs when people take antibiotics or other microbial drugs. It is an enormous problem in hospitals and chronic care facilities. Although C. difficile bacteria are found normally found everywhere, after taking antibiotics they grow to unusually large numbers in patients' intestinal tracts. In the U.S. alone, 5,000 deaths are caused by this disease, every year.
For clinical info, see: http://www.aboutibs.org/Publications/CDifficile.html
more info: http://www.mayoclinic.com/health/c-difficile/DS00736