Colorado has widespread infestations of Giardia. Crypto is not very common, but does occur here. I know of several people who have contracted Giardia from seemingly clean high altitude water sources including some on the CT. It is spread by mammals, and there are lots of those in the mountains.
I carry one full quart bottle and a filter plus a second empty bottle for camp or dry stretches, of which there are few. When I hit a water source I filter and drink a large quantity then fill my bottle for the next stretch. I am always drinking delicious water and the overall weight carried is actually less than if I carry treated water while I wait for the chemicals to be effective. It takes a minimum of 1 hour to chemically kill Giardia and up to 6 hours to kill Crypto. My MSR filter weighs 11 ounces and a full 32 ounce Nalgene weighs 38 ounces full, so I am actually carrying less weight with the filter than I would be with the tablets.
Some people don’t use any treatment method and don’t get sick. Others trying this method do get sick and it’s a good way to ruin a hike. I’ve had Giardia several times and although it has never stopped a hike it’s certainly not much fun to have and it’s expensive to treat.
bearcreek