Infection with toxin-producing strains of Clostridium difficile is common and potentially life-threatening. It occurs mostly in patients in the hospital or nursing home who are taking or have recently taken antibiotics. Two toxins, A and B, damage the colonic mucosa, resulting in symptoms ranging from mild diarrhea to bloody diarrhea with fever and abdominal pain, colitis, or even pseudomembranous colitis. Severe cases may involve dehydration, toxic megacolon, or colonic perforation. This article reviews the microbiology, epidemiology, clinical manifestations, diagnosis, treatment, and prevention of this disease.