There are a lot of animals besides beaver that carry giardia. It's true that most high mtn water is clean. Most. This whole water treatment thing is just playing the odds of a stream being infected. The higher you get, the lower the odds. However, the odds rarely get down to 0.
The thing is, there's a certain % of streams that are infected and there's no way of knowing which ones they are. It's a low % to be sure. Drinking from them is playing roulette. The more you do it, the greater the chance that you'll get one of the infected ones.
