An illness in which someone develops more watery and frequent stools than is typical for that person. Diarrhea can be caused by changes in diet, such as drinking excessive amount of fruit juice, eating more than the usual amounts of certain foods, and the use of some medications. Diarrhea also can be the result of a problem with the intestines, such as inability to absorb nutrients or allergy to foods. Infections with some viruses, bacteria, and parasites can cause diarrhea.