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. It is due to a lack of an enzyme in the intestine which digests milk sugar or lactose. The problem is caused by a deficiency of the enzyme lactase. The cure is simple - don't drink cow's milk. Unfortunately drinking cow's milk is so ingrained in our society that some individuals have a hard time giving it up.