To elaborate on that, Sendhair, Giardia (as I’ve heard it called) is not fatal, nor is it incredibly dangerous. It will make you violently ill for a few days and I think it can put you in the hospital, but I don’t think you could die from it. In fact, most wild animals live happily with the giardia parasite. Furthermore, I once met a guy, who had done the Appalachian Trail (2,000 mile hike) 7 times. He got giardia at some point and just decided to keep it. Now, he can drink from mountain streams whenever he wants, because he’s reached a sort of equilibrium with the giardia.
Please don’t take this as my promoting drinking from mountain streams or for contracting any parasite. I could be wrong about all this.