Clostridium difficile is a bacteria that grows in the absence of oxygen. It's ubiquitous in nature and is naturally part of the bacteria that live in the intestines of about 2-5% of adult human beings. C. difficile is cast off in feces and can create spores that can persist in the environment for lengthy periods of time waiting for a new host. Under most circumstances, other bacteria in the intestines can keep c. difficile in check so that it does not proliferate. However, in rare circumstances that only exist in hospitals, they may become the dominant bacteria in the intestines and this will often lead to diarrhea and colitis that in severe cases can lead to perforation of the large intestine, sepsis, and death.
Particularly at risk are the elderly, anyone who has been in a hospital environment for a lengthy period of time, individuals on high dose antibiotics (which generally do not affect c. difficile) and persons taking proton-pump inhibitors.
The first line of defence is good hygiene. The spores are easily transmitted on the hands of health care worker and can be transmitted if someone touches a contaminated surface, then touches something that eventually reaches the mouth of a patient. In a healthy adult, the spores can be overcome by stomach acid alone and, as noted, will generally not be able to complete with normal bacteria in the gut.
An infection should be treated as a medical emergency. The first line of defence is rehydration and, in many patients, this alone will allow recovery. However, narrow-spectrum powerful antibiotics have to be used for persistent infections.