sandia means this article here This is the oft cited article on the other side that I have seen.

I freely admit that I now usually treat my drinking water. However, I put on a couple of thousand miles with my family in the Sierra Nevada and Mt. Lassen from 1966 through 1979 without treatment generally speaking. If we had any doubt, we boiled the water. But this was rare in the Sierra and usually we just chose wisely. Giardia was around before I was born...so what was the difference between then and now? The amount of human traffic in the mountains is possibly the only major difference.