Odds are, a great many of them are sick. Waterborne illness is still one of the leading causes of death on this planet. And some people just build up a tolerance or are lucky- I can think of a couple of places I've been where people had wells or springs and they were contaminated. People got sick, got better and thought it was the flu, or they toughed it out for life. Either way, they became carriers of giardia.

Other things, like shistosomiasis and crypto, really don't have symptoms until you are boned. The first doesn't happen in the US, but if you are traveling or someone upstream has been traveling and you've got the worst luck in the world, it probably won't kill you, but you aren't going to be going anywhere for a while. If you need to MOVE NOW, that isn't something you want to have to deal with. Giardia and crypto can kill you by making you spend too much time squatting over a latrine, the same way cholera and dysentery kill. Again, if you need to evacuate on foot, passing 8+ quarts of water a day (crypto and giardia can both do that) is going to at least slow you down. Assuming you can rehydrate.

Cholera is almost unheard of in the US; the last significant outbreak was in the early nineteen-teens. But common in many parts of the world, and once you are a carrier, you are a carrier for life. Last major outbreak I know of in this hemisphere was 95-97 in Central America. Mexico STILL has pockets of it breaking out on a somewhat regular basis. We lucked out in NOLA- this nasty little thing is one of disaster's two ugly step sisters (along with typhoid) in a global sense. This can kill in a matter of hours as you try move more water out of your backside than is in your body.

Sometimes, mobility is life. Bad water usually gives you the runs. If you can't get it under control, all you are doing is running to a grave. I'm saying this because sometimes, you don't get a chance to see a doctor a few days later. I don't know how many times I've read people say things like "I'll risk it and see a doc later", but there is almost always a way to purify or at least filter water.