Diarrhea is defined as loose, watery, and frequent stools. Diarrhea is considered chronic (ongoing or prolonged) when you have had loose or frequent stools for longer than four weeks.

Diarrhea in adults is usually mild and resolves quickly without complication. In infants and children (especially under age 3), diarrhea is more concerning because they can become dehydrated fairly quickly.

The most common cause of diarrhea is a mild viral infection that resolves on its own within a few days. This is called viral gastroenteritis or "stomach flu." Viral gastroenteritis often appears in mini-epidemics in schools, neighborhoods, or families.

Food poisoning and traveler's diarrhea are two other common causes of diarrhea. They occur from eating food or drinking water contaminated with organisms like bacteria and parasites.

• Malabsorption syndromes -- conditions in which you are unable to absorb or digest certain nutrients. Common examples include lactose intolerance, celiac disease (also called sprue or gluten malabsorption), cystic fibrosis, and intolerance to the protein in cows milk or other specific foods like beans or fruit.
• Inflammatory bowel diseases which include Crohn's disease and ulcerative colitis -- two recurring types of diarrhea that are generally bloody and accompanied by abdominal pain.
• Irritable bowel syndrome (IBS) -- a chronic form of stomach upset that gets worse from stress; if you have IBS, you generally have alternating diarrhea and constipation.
• Immune deficiency.

Medications can also cause diarrhea, especially antibiotics, laxatives containing magnesium, and chemotherapy for cancer treatment.

• Zollinger-Ellison syndrome
• Nerve disorders like autonomic neuropathy or diabetic neuropathy
• Carcinoid syndrome
• Following gastrectomy (partial removal of the stomach)
• High dose radiation therapy