I think the answer to that is really on a case by case basis. Some people's bodies just simply can't handle it, even after they've had it, maybe that's me. We'll find out in the future if I get it again.
The locals, at least some of them, do drink the tap water, they've grown up with it and their bodies seem to handle it just fine.
Myself, not being used to it, clearly cannot handle it at all.