From a clinical perspective, diarrhea 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 diarrhea per year, of which 900,000 result in hospitalizations and 6000 in deaths. Worldwide, there are 2 billion episodes of diarrheal disease per year, of which 1.9 million resulted in death among children <5 years.  This makes it the second leading cause of death in postneonatal children <5 years, after pneumonia.  
Normally, approximately 10 liters 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 reabsorption. Overall, about 99% of the fluid is reabsorbed, leaving 0.1 liter to be excreted in the feces. Diarrhea 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 diarrhea, 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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