From a clinical perspective, diarrhoea can be defined as the passage of: 
three or more loose or liquid stools per 24 hours; or
stools that are more frequent than what is normal for the individual.
acute (≤14 days);
persistent (>14 days); or
chronic (>30 days).
In the US, there are 375 million episodes of diarrhoea per year, of which 900,000 result in hospitalisations and 6000 in deaths. Worldwide, there are 2 billion episodes of diarrhoea per year, of which 1.9 million resulted in death among children <5 years.  This makes it the second leading cause of death in post-neonatal children <5 years, after pneumonia.   Implementation of improved access to oral rehydration therapy and education on feeding and weaning practices should help to reduce mortality from diarrhoeal disease, especially in the under-5 age group. 
This compares to England and Wales, where infectious intestinal disease causes 300 deaths and 35,000 hospital admissions annually.  There are an estimated 17 million cases and 1 million consultations with a general practitioner attributed to acute infectious diarrhoea in the United Kingdom every year. 
Normally approximately 10 litres of fluid consisting of ingested food and drink, in addition to secretions from the salivary glands, stomach, pancreas, bile ducts, and duodenum, enters the gastrointestinal tract every day. The small intestine is the major site for re-absorption. Overall, about 99% of the fluid is re-absorbed, leaving 0.1 litre to be excreted in the faeces. Diarrhoea occurs when various factors interfere with this normal process, resulting in decreased absorption or increased secretion of fluid and electrolytes, or increase in bowel motility.
Improved understanding of the pathophysiology of infectious diarrhoea, and the factors that promote the spread of causative infectious agents, will lead to practical approaches for preventing and responding to outbreaks. 
Colorado Infectious Disease Associates
SP declares that he has no competing interests.
Dr Sean Pawlowski would like to gratefully acknowledge Dr Mamoon Elbedawi, Dr Peter Draganov, and Dr Cirle A. Warren, previous contributors to this monograph. ME, PD, and CAW declare that they have no competing interests.
Honorary Clinical Research Fellow
Department of Biosurgery and Surgical Technology
St Mary's Hospital
GER declares that he has no competing interests.
Senior Medical Resident and Clinical Fellow in Nutrition and Gastroenterology
Department of Gastroenterology
Beth Israel Deaconess Medical Center
DAL declares that he has no competing interests.
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