When you have a piece of outdoor clothing that is supposed to be waterproof, you can easily know if it is reliable, just go and walk in the rain. With personal water filtration systems it is far different. There is no way to know.
Using a Water Purifier in the Mountains – Read Why You Should
Numerous ancient viruses have been found in a Tibetan glacier, many thousands of years old, and some of them have never been seen before.