Diarrhoeal disease is any disease caused by bacteria, viruses, or parasites that has diarrhoea as its main symptom — one of the great categories of food- and water-borne illness. The roster includes cholera, typhoid, shigellosis, campylobacteriosis, salmonellosis, rotavirus and norovirus infection, giardiasis, and amoebiasis, all spreading through the faecal–oral route: contaminated water, unwashed hands, unwashed produce, undercooked food. The danger is dehydration — rapid loss of water and electrolytes that can kill infants and the frail within hours — which is why oral rehydration salts are among the most life-saving interventions ever devised. Globally the burden falls heaviest where clean water and sanitation are scarce. For food safety, diarrhoeal disease is the outcome the whole system exists to prevent, from farm hygiene to the chlorination of drinking water.