It's funny, but nature just has it ways of having surviving. When we have the knowledge of where our water comes from, then I don't think we would even want to drink them at all. There are lots of bacteria in water both pathogenic and non pathogenic, water goes through a lot of processes before ending up in our cup.

I still feel psychology has its own way of working on us. There is a saying in Nigeria that goes

water no dey kill black man.

Meaning want doesn't kill a black man. And some people have work with that ideology even though it's wrong and it worked for them.

Even though scientifically it might be because of their immune system. As you stated in your post too, it might be that you had a better immune system than the other kids. Thanks @reddust. From yours' loyalist @tobex.