Cryptosporidium and Giardia are the most likely contaminant in wilderness areas of the US, but bacteria and viruses can make cause more serious and chronic illness. When I really had to, I treated cool water with a double does of Aquamira drops and waited 40 minutes, but that was a risk necessitated by pushing on instead of stopping to treat the water when I should have. The taste of the double dose made me wish I had just risked getting sick.
The water in the areas I frequent is almost always cool enough that I'm not confident unless I treat for 4 hours with a single regular dose of Aquamira drops. I don't really have time to wait 4 hours. I don't want to carry the extra water around either. I concluded that the best approach for me was to get a pump filter (actually a First Need purifier) so that I didn't have to mess with either
chemical treatment, boiling, or gravity filter setups. Chemical treatments taste worse than city water to me. It seems like such a shame to have to taint good-tasting mountain water before drinking it. The filter weighs around 19 oz with residual water, which is less than the minimum extra liter of water (2.2 lbs) I'd have to carry if I had to wait 4 hours for chemical treatments to work.