Table 1. Definitions of diarrheal illnesses.

Diarrhea	The most commonly recognized definition of diarrhea is based on World Health Organization parameters and define diarrhea by the passage of 3 or looser than normal stools in the preceding 24-hour period. An episode of diarrhea is defined as lasting 1 day or more and usually ends after at least 2 days without diarrhea.
Acute diarrhea	Episode of self-limiting diarrhea with acute onset, typically lasting 5 to 7 days. In most cases, it is due to an intestinal infection and may be combined to fever and vomiting, meeting the definition of acute gastroenteritis. Acute diarrhea may be also related to extra-intestinal infections (i.e. urinary infection, viral respiratory infections), food-poisoning, iatrogenic intestinal damage (i.e. chemotherapy, radiotherapy) or other intestinal and extra-intestinal diseases such as acute appendicitis.
Prolonged diarrhea	Acute onset diarrhea lasting from 7 to 14 days not covering the definition of persistent diarrhea. It is usually due to persistent infections or to post-infectious intestinal damage (i.e. carbohydrate malabsorption, small intestine bacterial overgrowth) that may prolong the duration of diarrhea behind the expected time. Some experts refer to this as acute-protracted diarrhea.
Persistent diarrhea	Diarrhea lasting 14 days or more, usually associated with weight loss, ultimately leading to severe nutritional impairment and that may require clinical nutrition. The classical definition of persistent diarrhea was intended to exclude some causes of chronic diarrhea such as celiac disease or inflammatory bowel diseases.
Chronic diarrhea	In many contexts chronic diarrhea is a synonymous of persistent diarrhea. The World Health Organization uses this definition rather than persistent diarrhea. However, chronic diarrhea usually does not have an acute onset and is the manifestation of structural and inflammatory bowel disorders. Some experts refer to chronic diarrhea in case of episodes lasting more than 4 weeks.
Post-infectious diarrhea	Acute onset diarrhea lasting 7 to 14 days and following an episode of acute gastroenteritis. This definition is covered by prolonged diarrhea.
Intractable diarrhea	Non-infectious diarrhea lasting more than 14 days, intractable despite extensive hospital therapy. Typical of young infants, usually below 3 months (but not only). Typically needs intravenous fluids or clinical nutrition and is related to high mortality.
Congenital diarrhea	Congenital diarrhea is an inherited enteropathy with a typical onset early in life. For many of these conditions, severe chronic diarrhea represents the main clinical manifestation, while in others, diarrhea is only a component of a more complex multi-organ or systemic disease.