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Acute diarrhea, defined as the presence of three or more loose stools per day for less than 2 weeks, is usually self-limited and infectious in etiology. In contrast, chronic diarrhea has a duration of more than 3–4 weeks, is less likely to resolve spontaneously, and is more likely to be mechanical in origin. In general, the pathophysiology of diarrhea is osmotic, secretory, inflammatory, or mechanical. Osmotic diarrhea is the result of poorly absorbed molecules, such as lactulose, that draw water into the intestinal lumen. Inflammatory diarrhea occurs when inflammation of the bowel mucosa causes decreased fluid resorption. Secretory diarrhea occurs when there is an increased amount of fluid secreted into the bowel lumen, usually secondary to the effects of bacterial enterotoxin or other secretagogues on the mucosa. Mechanical diarrhea occurs with increased gut motility and is often seen in irritable bowel disease or following surgery. Acute infectious diarrhea is generally inflammatory, secretory, or a combination of both.
Infectious diarrhea is among the leading causes of adult mortality worldwide and is the single leading cause of childhood mortality, resulting in the deaths of 1.6 to 2.5 million children every year. In the United States, as many as 375 million episodes of diarrheal illness are estimated to occur annually. Diarrheal illness is responsible for approximately 5% of U.S. emergency department (ED) visits, as well as 900,000 hospitalizations and 6000 deaths each year.