If you’re a citizen of the world, you’ve probably met up with that uncomfortable phenomenon known as traveler’s diarrhea—a short episode of diarrhea that strikes thosebrave enough to visit local places and enjoy local cuisine. The odd part is that traveler’s diarrhea seems to affect only travelers. Native people can eat the same foods and emerge unscathed.
The first thing to understand is that traveler’s diarrhea needs a certain level of sanitary sloppiness to occur. In particular, it only happens if there’s some way for bacteria to pass from another person’s (or an animal’s) feces into your environment. For this reason, traveler’s diarrhea happens much less often to visitors of most first-world countries. (Although Mexicans do occasionally get diarrhea when visiting the U.S., which they call“Washington’s Revenge.”)
However, this doesn’t explain why locals have a much-reduced rate of diarrhea. The answer is that, because of near-continuous exposure, their digestive systems have gradually grown to recognize and tolerate strains of bacteria that other people can’t handle. No one knows how long this immunity takes to develop or how long it holds up, but a study in Nepal found that American adults needed 7 years of local life to adjust, and they lost their tolerance after only a few months back home.
Interestingly, enterprising travelers can use one approach for instant immunity. If you’re worried about E. coli (which is the most common culprit in Mexico), you can buy a vaccine called Dukoral that gives you temporary immunity. To get Dukoral, head to your local pharmacy or check with a travel clinic, which can also identify the gastrointestinal dangers in different parts of the world.
Source of Information : Oreilly - Your Body Missing Manual