Diarrhea can be a symptom of many different illnesses and medical conditions, or a side effect of certain medications. Diarrhea may also be the result of specific dietary choices. Potential causes of diarrhea include:

• Viral infection. Viruses are the most common cause of infectious diarrhea. A virus may damage the lining of the small intestine, interfering with normal fluid and nutrient absorption, causing diarrhea. Rotavirus is the most common cause of severe diarrhea in children – an estimated 55,000 children are hospitalized for it each year in the United States, according to the Centers for Disease Control and Prevention (CDC). Viral infections can spread easily. Noroviruses cause gastroenteritis and intestinal flu, which include diarrhea as a primary symptom.
  
  
• Bacteria/parasites. Bacteria (e.g., Escherichia coli, salmonella) and parasites (e.g., giardia) are commonly ingested through contaminated food or water. Some bacteria and parasites release toxins that cause the intestines to secrete water and salt, adding to the liquidity of stool.
  
  
• Intestinal disorders. If the colon is inflamed or diseased, it may be unable to absorb fluid from food products traveling through the intestines, resulting in diarrhea. Examples of intestinal disorders that can cause diarrhea include inflammatory bowel disease (IBD), celiac disease and dysentery.
  
  
• Motility disorders. Problems with the movement of stool through the digestive tract can also lead to diarrhea. When stool moves too quickly through the intestines, not enough fluids are absorbed from the stool, leading to watery, unformed bowel movements. If stool moves too slowly, it can lead to bacteria overgrowth and loose, greasy bowel movements. Causes of motility disorders include an overactive thyroid, irritable bowel syndrome (IBS), scleroderma and antacids that include magnesium. Stress and anxiety can also cause increased movement of stool through the digestive tract, resulting in diarrhea.
  
  
• Surgical procedures. Various types of surgery may cause changes within the digestive tract that can lead to diarrhea. Diarrhea can be caused by dumping syndrome, a common consequence of gastric surgery. Bowel resections decrease surface area of the intestines, which limits fluid absorption and may cause diarrhea. Bacterial overgrowth as a result of Crohn's Disease or intestinal surgery can cause inflammation and diarrhea. Surgery to remove the gallbladder can speed the movement of stool through the intestines, causing diarrhea, but this is usually temporary.
  

• Medication. There are hundreds of prescription and over-the-counter medications that can cause diarrhea. Antibiotics are the most common culprit – they can disrupt the natural balance of bacteria in the intestines. Diarrhea is also a common side effect of medications used to treat depression, high blood pressure and cancer. Drugs and synthetic foods used to promote weight loss (e.g., foods containing the fat substitute olestra) often cause diarrhea. Habitual use of laxatives can also cause diarrhea. Research also indicates that use of illegal drugs (e.g., crack cocaine) can cause diarrhea as well. In most cases, diarrhea resolves once the antibiotic or other aggravating agent is discontinued.
  
  
• Artificial sweeteners. Products used as sugar substitutes, such as sorbitol, are sometimes not absorbed into the bloodstream and remain in the intestines, leading to loose, watery stool.
  
  
• Caffeine, alcohol. Caffeine and alcohol stimulate the passage of stool, which can cause it to move too quickly through the intestines, disrupting normal fluid absorption and resulting in watery stool.
  
  
• Lactose intolerance. People who lack the enzyme lactase are unable to digest milk products. The undigested lactose in the intestines causes an osmotic diarrhea (diarrhea caused by problems with the absorption of carbohydrates).
  
  
• Ingestion of toxins. The inadvertent ingestion of environmental toxins (e.g., insecticides, arsenic) can lead to diarrhea. Eating poisonous mushrooms may also cause diarrhea.