The fact is, some people are resistent or become resistent against some kind of virus in the areas they live.
I have travell to the US and I did not drink tap water anywhere. There is no filters attached to the taps, and nobody can guarantee that the water box of the building is clean. Actually, I usually avoid drinking tap water in any country that I have visited, including UK and Europe, for the same reason. I am not posh, but I go for bottle water when travelling for a safty measure.
In Brazil, the water is potable. In many south america countries the water it is also potable. Only in restricted areas there is no water treatment. Anyway people use to have a filter between the pipe and the tap, where they get the water to drink. But in many cases it is not necessary. Some people drink water directely from the tap.
Last year I was in Peru and I think the water was potable in most of the cities, otherwise the population would be all sick. Even avoiding drinking water from the tap I think I had a problem with rotavirus because I ate some grapes that I washed them using tap water in the day before. Or may be it was because someone had put dirty hands on the grapes and contaminated it. It was not a big deal, I was just sick for a few hours, there was not need to go to hospital. Maybe the water it was potable to the locals, but not to me. Actually, the more "civilized" or cautious people are, the less weak they became. (It was also said by "Dr House" in the famous TV series)
The progress of the civilization does make people stronger before the nature, people are getting weak day by day.