About 2-5% of humans carry a low population of Clostridium difficile in their intestines; these people apparently do not get sick because the other bacteria that comprise their normal gut flora compete with the C. difficile and keep it in check. However, these people do shed C. difficile endospores in their feces, and such endospores can commonly be found in a variety of places such as health care facilities.
When someone takes antibiotics the drugs frequently wipe out a large part of the normal gut flora. If one is exposed to C. difficile endospores at such a time, the spores may germinate, and the C. difficile bacteria can multiply very rapidly, having no competition to slow it down. C. difficile releases an exotoxin that can cause violent diarrhea; additionally the bacteria itself can completely overgrow the lining of the gut, causing a life-threatening condition known as enteritis necroticans.