Much of the problem with "tummy bugs" when abroad is due to not being used to the local germs. You don't have the right intestinal flora, the right antibodies etc. You can eat the same food as the natives and you'll get sick and they won't.

There are limits, though. When people like Oxfam campaign for water in 3rd world countries, they are often campaigning for clean water. Water from a well is drinkable because it has, in effect, been filtered by passing though rocks on its way to the water table.