I'm with MountainWalk, I haven't used my water filter in 8 or 9 years, and I've never been sick from anything that I eat or drink in the Mt's. When I was in high school and would be out changing sprinkler pipes that were filled with irrigation water from the Yakima river, I'd get a little thirsty and I'd drink all I wanted. The reason the Indians got sick from the white explorers/settlers as they moved west is because they were being exposed to "germs" that they're bodies had never dealt with before, so they had no resistance.
About 10 years ago, I read an article, I believe in Outside magazine, about this very subject. In it, they took water samples from all over California, from heavily visited areas of Yosemite park, to mountain streams and lakes in the Sierra Nevadas. I do not remember the exact numbers, but I do remember feeling justified by the results of their tests. Only the most obviously contaminated water had bacteria in it, in high enough numbers to make the average person sick. So, ever since then I have felt very comfortable drinking water from nearly any source in the mountains.
Scott