As long as there is a source of clear flowing water and the area is not heavily polluted it may be completely acceptable to drink from a stream. People have been doing it since the dawn of time. Filters have only been around for a very short time. Even fire is something that humans have only mastered fairly late in the evolution process and honestly, boiling just isn't always practical.

Don't get me wrong - I'm all for playing it safe, boiling, chemically treating or filtering water whenever possible. But sometimes I think people overestimate the dangers. The outdoors is not necessarily such a hostile place. However, it sure pays to avoid stagnant water and any source close to densely populated residential or farming areas.

I've travelled a fair bit and it always amazes me how the locals in developing countries regularly drink water that would be considered totally unsafe by our standards. No filters, no boiling, yet they still manage to get by. I think it's also a matter of getting used to it. If you are exposed to bacteria all the time your body will develop a natural resistance. That's probably the reason why tourists get sick all the time in third-world countries though the locals have no problem with the food and water.