For the bigger part of my life, I had no knowledge about water treatment.
So I've travelled long and wide, the Middle East, North Africa and USA/Mex drinking about everything I've ever got, untreated - without any more problem than occassionally some light diarhhoe.
Once in Israel I've developed a very serious sickness that kicked me off the bike within minutes, shivering in fever and writhing on the ground (I'll not give further details), but this was at a time when I lived in civilisation drinking botteled water, and the real cause I never got to know.
OK, this was when I was young and strong and adventurous. And true, there were by far not as many people everywhere as there are nowadays.
I strongly belive that people's waste is the most dangerous.

Nowadays, I take a very close look at the map (or Google Earth) to get a clear picture where the water I'm going to get comes from.
If there is a settlement or any other kind of civilisation upstream, I will treat the water.
If this is a cold spring and no hint of civilisation upstream, I might just take it as it is.