Clostridium difficile is now regarded as the most prevalent nosocomial pathogen, infecting as many as a quarter of hospitalized patients. The pathophysiology of infection with this unusual enteric pathogen involves alteration of the normal enteric flora by antibiotics, ingestion of spores, and colonization by C. difficile. The organism then releases potent exotoxins that produce an inflammatory colitis and diarrhea. A spectrum of host responses occurs, ranging from the asymptomatic carrier state to life-threatening pseudomembranous colitis. Effective therapy with vancomycin or metronidazole is available, but relapses occur in 15% to 20% of patients and may necessitate multiple courses of therapy.