Diarrhea is a clinical symptom characterized by an increase in the volume, wateriness, or frequency of bowel movements. Acute diarrhea is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause severe dehydration which is one cause of death in diarrhea sufferers. The causes of diarrhea can be infectious or non-infectious. Infectious causes are due to bacteria, viruses, or parasites. This book presents research on diarrheal viruses; cholera in Africa; the pathogenesis of Clostridium-difficile associated diarrhea; fecal organic anions in calf diarrhea and the pathophysiology of irritable bowel syndrome with diarrhea.