Also known as Montezuma's Revenge; it can be caused by either bacteria or amoeba. The latter is the fiercer and long-lived of the two.

How does the disease spread? Well, it could be in the water, and even if you are carefully not drinking water, you may be carelessly using ice made from infected water. The infection travels via human feces (ugh), and when food is infected it is because an infected person has not washed his/her hands after doing number two.

Certainly none of this information is of any consolation once you are doubled over, running for the bathroom, but it might help impress the need for cleanliness in eating establishments.