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.
Actually, diarrhea is the NUMBER ONE cause of death for children, world wide. Almost always due to compromised water.
Cholera, just as a tidbit of info, can cause you to lose up to 30 liters (might be pints, I'm pulling from memory) of water daily. Given the average adult male is only about 60L of water in them, you can see why it'll kill quick.
Regarding field expedient filters, I don't think ANY made up filter can get down to the microscopic level. I did read somewhere, maybe Discover magazine, that most of the water in the Sierra Nevadas is clean enough to not need filters, sort of. Some college professor had sampled dozens of lakes, and in the first few inches deep, enough UV light from the sun did a good job of keeping it relatively clean. I think if worst came to worst, a very gentle slurping of the top of a good size lake would be a safer bet than most other options. Or collecting rain, but we all know how often CA sees THAT.
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