When I was in training to become a SERE Specialist I was sent on a second solo evasion living exercise because I did terrible at my first one. The weather wasn’t terrible and I mostly had sun. The temperature dropped well below freezing at night but I was allowed to have a fire. Life was quite peaceful and I settled into the beautiful mountainside that I had made my home. There was a large meadow perhaps 50 meters from my camp that had an ice cold meandering stream running through it. Disregarding my training because I’m an idiot I looked at the mountains surrounding me, jutting up into the clouds and thought, “This here is pure mountain water, purified by the soil and cleansed by sunshine.” And I happily drank without purifying.
Day 3 of my exercise I collected enough water to make a small journey to see if I could find some materials. I followed the “pure mountain stream ordained by the sun” upstream for around 500 meters before I came to another meadow equally as beautiful as the one I lived next to. I heard a noise moments later and watched a herd of perhaps 80 or so cows cross the stream and shit into it. I was lucky. Purify your water.
In a survival situation you should purify all of your water. Even rainwater as it’s descending to you can hit the tree canopy and collect fecal mater from various animals that live in trees. There are various methods to purify water that include the use of purchased products and ones that need nothing but your ingenuity.
Pre-filtering you water: This is important, if your water is full of particulate matter and other disgusting crap you might want to consider doing a pre filter. I typically do this by putting a shirt over the mouth of my canteen and pouring the water through my shirt. This limits the amount of debris getting into your water and lessens the chance of you getting a water-borne pathogen.