Here is an excellent article on Giardia, written by a certain fellow named R. Rockwell. (Thank you Bob.) Pay particular attention to the section regarding the Sierra's. I've never had the malady, drinking unfiltered, clear water, but I am careful to filter drinking water in the area below Bishop Pass (it's become one of the most trashed trails on the east side).
http://www.ridgenet.net/~rockwell/Giardia.pdf
