Diarrhea is the passing of loose or liquid stool, more often than normal. Diarrhea can be caused by diseases of the small intestine, large intestine or by diseases of organs other than the intestinal tract. Your ability to answer questions about your pet's diet, habits, environment and specific details about the diarrhea can help the veterinarian narrow the list of possible causes, and to plan for specific tests to determine the cause of diarrhea.