Well, maby some people like giardia, naglaria, parasites, worms and various bacteria that make your pants brown? Also chemicals (pollution) and pesticides is a factor you have to consider before drinking.
I did an extensive writeup on water purification earlier:
http://tonfiskpaburk.blogspot.se/2016/11/getting-drinkable-water-bushcraft.html
envirozentinel wrote:
Mountain water above human habitation is usually safe to drink, such as in wilderness areas.
Cholera and all those other nasty bugs are usually found where there are people who pollute the water with all manner of things I prefer not to think too deeply about...
It CAN be safe to drink, but you should always check upstream before. A "little" chemical pollution wont kill you but i would never gather water from a ditch nearby a farm. Giardia and similar ailments aren't extremely lethal, but instead you will dehydrate from diarrhoea and if you're out in the wild, you're done for. That is why you boil water or run it through a filter.
A sawyer mini costs $30 here in Sweden, probably cheaper elsewhere. Get one and keep it on your person (they freeze = they break). And even a monkey can be trained to boil water.
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"It is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring" (Carl Sagan)