Diarrhea is a liquid stool or bowel movement that results from the movement of fecal material through the intestines in such a rapid manner that there is insufficient time to properly absorb water. In addition to excessive water, diarrhea waste contains abnormally high amounts of electrolytes. According to World Health Organization statistics, diarrhea--resulting from a number of causative agents--is the leading cause of dehydration and death in children.

If excessive amounts of the electrolytes are lost as a result of diarrhea, the loss interrupts the normal acid-base balance (e.g., excessive potassium loss induces acidosis).

Diarrhea may be acute (rapid onset, short duration) or chronic (occur over a long period of time). Many infectious bacteria such as Shigella, Salmonella, and Campylobacter may induce diarrhea. In many areas of the world with poor water quality, parasites such as Giardia and Cryptosporidium are leading causes of diarrhea. Viral infections, especially rotavirus infections, also cause diarrhea.

Diarrhea-inducing agents generally produce an irritation of the gastrointestinal tract, specifically the gastrointestinal mucosa. Other leading causes of diarrhea include ingestion of spoiled food laden with harmful bacteria (food poisoning). Diarrhea may also result from noninfectious causes including overeating, milk intolerance, or a number of bowel and/or intestinal disorders. Diarrhea may also occur as an unintended side effect of medical treatment with antibiotics or other drugs.

Any increase in peristalsis—the wave-like, directional contraction of longitudinal and smooth muscle in lining the intestine that helps food and waste move through the intestines--may result in diarrhea. Accordingly, emotional disturbances that result in increased peristalsis may induce acute diarrhea.

Types and the extent of diarrhea are classified by whether the diarrhea is acute or chronic, the volume of the fecal material expelled, the frequency of expulsion, the water content of the stools and whether mucus or blood are present in the stool.

When diarrhea occurs without vomiting (the expulsion of food and water through the mouth) it is often possible to rehydrate individuals suffering from diarrhea while attempting to treat the actual cause of the diarrhea. Children are often rehydrated with osmotically balanced liquids such as Pedialyte. The World Heath Organization uses Ricelyte, or it's own osmotically balanced rehydration formula to replenish lost water and needed electrolytes in dehydrated patients.