Basic Cause

• Diarrhea is a watery stool that is likely to pass quickly and may cause you to feel a sudden and urgent need to defecate. Diarrhea occurs when the colon is unable to absorb enough liquid from the foods and fluids you eat before they pass through it. This can happen when a large volume of food and liquid pass through the colon, making it difficult for it to absorb enough fluid to make the stool reach a semisolid consistency, or the food may pass through the colon too quickly for proper absorption. Conditions that affect the ability of the colon to absorb fluids may also lead to diarrhea.
  
  

Illness

• Diarrhea commonly occurs during illnesses that affect the ability of the stomach and colon to process food and liquid. Viruses like the Norwalk virus, herpes and hepatitis can cause diarrhea, and tend to be easy to spread from one person to another. Bacterial infection can also lead to diarrhea; diarrhea-causing bacteria often gain access to the gastrointestinal tract trough contaminated food and water. Diarrhea is especially problematic in poor countries, since water and food are more likely to contain bacteria and parasites. E. coli is a one of the more well-known types of food poisoning bacteria that can cause diarrhea.
  
  

Medication

• Taking medication and other drugs can cause diarrhea since they can upset the normal balance of chemicals and bacteria in the body. The intestines naturally contain certain types of bacteria that can be useful for digestion. If you take antibiotics designed to fight a certain bacterial infection, they can interfere with the good bacteria of the colon and cause diarrhea. Other treatments that alter the normal balance of the body, such as chemotherapy for cancer, may also lead to diarrhea.
  
  

Other Causes

• The foods and drinks you consume play a large role in whether you will develop diarrhea. Certain people have difficulty digesting certain types of food--food intolerances such as lactose intolerance can lead to diarrhea. Some people are susceptible to diarrhea when they consume fructose, the sugar contained in fruit, and others may suffer diarrhea from artificial sweeteners.
  Irritable bowel syndrome and inflammatory diseases of the colon such as Crohn's disease can cause diarrhea. Surgery performed on the stomach or colon may also lead to diarrhea as it can disrupt the ability of the body to properly digest food.