Diarrhea is an increase in the liquidity and/or volume of stool or the frequency of bowel movements. It may be mild, moderate or severe and can have an impact on normal activities. The average frequency of bowel movements and firmness of stool can vary greatly from person to person. Thus, it is important to note any significant changes in what is normal for a particular person. In patients with diarrhea, stool can range from soft to liquid in texture, brown to clear in color, and defecation may occur more than three times a day.

Diarrhea is caused by a disruption of the normal process of digestion. Normal digestion involves food traveling from the stomach to the small intestine, where most nutrients are absorbed. By the time it gets to the colon, only waste material remains. The colon absorbs most of the water from the waste and passes the stool along via muscle contractions (peristalsis) to the rectum at the end of the digestive tract. Diarrhea occurs when too much fluid remains in the intestines as stool travels through the digestive tract. When this occurs, the stool that is eventually passed through the anus is loose and watery.

Too much fluid in the intestines can be caused by several different types of digestive dysfunction, including:

• The small intestine and colon may fail to absorb enough nutrients and fluids from stool, making the stool loose and watery.
  
  
• The intestines may be stimulated to secrete water, salt or other fluids into the digestive tract, making stool runny. This stimulation can occur as the result of infection, disease or the presence of certain other substances (e.g., castor oil, excess bile acids).
  
  
• Food may pass through the intestines too quickly or in too large an amount to be properly absorbed. The rapid transit of stool through the digestive tract is a common cause of diarrhea.

Diarrhea is a common experience. Millions of Americans report problems with diarrhea every year. It is estimated that the average adult experiences diarrhea approximately four times a year, according to the National Institutes of Health (NIH). It may be difficult to identify exactly how many people have diarrhea, since most people do not seek medical treatment.

Acute diarrhea (lasting less than four weeks) is the most common type of diarrhea. According to the American College of Gastroenterology, it is the second most common reported illness in the United States, after respiratory infections. Most cases of acute diarrhea are caused by infections or antibiotics. Most episodes of acute diarrhea resolve spontaneously (without medications or with simple dietary changes).