As a vast majority of ...

As a vast majority of responders have indicated, one should NEVER "assume" water to be free of harmful pathogens; no matter its source or other characteristics. I've known of hikers & Mountain climbers, who thought it was "safe" to drink from a "clear running" creek or stream, & wound up getting badly infected. Why: Because an animal carcass or some other rotting was in the upstream source. Another bad/"costly" assumption - that was re-emphasized by a 2007 research field test, contracted for by WHO/UNICEF - is that any water once treated, was then considered totally/safely "potable": The results of that extensive field test, instead, found that some 40% of the presumed "safe/potable" water - sitting in holding/storage vessels on dining tables, in offices & elsewhere - were found to be "seriously re-infected"*! [NB: Their words, not mine!]. In my field work, I've tested even the linings, spouts, hoses & other internal/external components of presumably "safe" - often community-serving - water holding/dispensing stations, & found them to be seriously infected (from touching hands, air-borne pathogens & even wild animal/bird dropping sources). My point is: It doesn't take much to seriously infect non-saline water, and contrary to popular belief, nearly ALL the most common forms of water purification leave NO residual downstream protection against re-infection. One final point, in reference to the poster who suggested a need to "warm" raw water: UNLESS ONE CAN BRING WATER TO A BOILING POINT TEMPERATURE, VIRTUALLY NO PATHOGENS WILL BE ELIMINATED! This proves to be a serious factor for populations living at higher altitudes, since it takes much longer/more fuel for water to reach boiling points at higher levels. And, for "turbid" or cloudy water, it takes longer for any purification system to work; but just a pinch of fairly common Aluminum Sulfate will clear such water, quickly & thoroughly.