For quite a few years i carried nothing at all for water treatment and as far as I know, I never got sick. Most of this was in the desert Southwest.<br><br>I now use a Pur Hiker ($35) which rated well in comparative tests. It is quick and handy when you don't want to stop and take the time to boil. I still haven't got sick. I suspect that often the water out there, especially in really isolated places, is just fine, but the various agencies give very conservative advice, since a case of Montezuma's Revenge tends to spoil the national park experience. If ths is so, then anything you do - filter, chlorine, magical incantations - will seem effective. I have never seen a test of water filters that demonstrated their ability to clean highly contaminated sources. Boiling is best and doesn't cost big bucks.