> Exactly right. If you're plane crashed in the Sierras and your trying to survive until rescue can arrive, I can see you risking it (after all a lack of water

> Exactly right. If you're plane crashed in the Sierras and your trying to survive until rescue can arrive, I can see you risking it (after all a lack of water will definitely kill you, water with Giadarsis in it will just make you sick). But If you are hiking/camping and you have the opportunity to bring something that can filter out that stuff, you will avoid some seriously nasty bad times. It's a bit of a trade-off. If you're off camping for a long weekend and your purification system fails but you're in an area with a body of fast moving water, then you might be better off drinking from that source and hoping for the best because symptoms won't hit you until you're back in civilization. If you're stranded, and you're uncertain when rescue is going to come, then catching a parasite from a dirty water source can drastically reduce your chances of surviving until you're found.