Diarrheal diseases, which are especially prevalent in the de?veloping world, cause significant morbidity and mortality. Itis estimated that diarrheal diseases kill more than 6,000children per day in Asia, Africa, and Latin America. Themost important cause of death associated with these dis?eases is dehydration, and the most important treatment thatdecreases death rates is rehydration therapy (Guerrant &Steiner, 2005).In the United States, the epidemiology of diarrheal dis?eases is changing constantly. Water disinfection, pasteuriza?tion, and appropriate food packaging have decreased the in?cidence of diseases such as typhoid and cholera. However,importation of foreign foods, environmental and ecologicalchanges, and changes in diagnostic test modalities have ledto recognition of important new trends and outbreaks.TransmissionThe portal of entry of all diarrheal pathogens is oral inges?tion. Although food is far from sterile, the high acidity of thestomach and the antibody-producing cells of the small bowelgenerally serve to decrease the potential of pathogens. In?fection can occur when the infectious dose is high enough orif the food neutralizes the acidic environment. Decreasedgastric acidity with disruption of normal bowel flora (as oc?curs after surgery), use of antimicrobial agents, and the im?mune dysfunction of AIDS all decrease intestinal defenses.CausesThere are many bacterial, viral, and parasitic causes of diar?rheal diseases. Common causes of bacterial infection in?clude E. coli and Salmonella, Shigella, Campylobacter, andYersinia species. The most significant viral causes of diarrheaare Rotavirus, which commonly results in diarrhea in youngchildren, and Calicivirus (often called Norovirus), a virus as?sociated with outbreaks in long-term care facilities andcruise ships. Parasitic infections of importance include Gia?rdia and Cryptosporidium species and Entamoeba histolytica.