Clostridium difficile is a spore-forming gram positive anaerobic bacillus that is part of the normal faecal flora of about 3% of healthy adults. Colonization rates may be much higher than that in hospitalized patients and in newborns. It can occur at any age, but elderly adults and debilitated patients are most susceptible to disease. In such patients, even a short course of antibiotic therapy for prophylaxis or treatment of minor infections can lead to colonization and Clostridium difficile infection. The list of causative antimicrobials is extensive but, in general, any antimicrobial agent that can get into the intestinal lumen can, even in small amounts, induce disease.