It is usually caused by E. Coli bacteria, although virus infections are quite common. It may be associated with cramping abdominal pain. Diarrhea is mainly treated with the diet and antibiotics. Travelers' diarrhea can last from two to three days up to a week if it is not treated. This is the danger of eating from street vendors.
Diarrhea can also result from parasite infections which are quite common in developing countries.
Amoeba and Giardia are common, if not universal in many countries. It can be associated with blood in the stool and severe cramping. Some individuals will have only one day of problems with the symptoms clearing for several weeks before recurring. Parasitic infestations can last for months and years. Lastly, if you have a bad episode of diarrhea, or an infection with Giardia or Amoeba, you may develop an intolerance to milk which is usually temporary but can be permanent.