Most filter water bottles remove 99.99% of pollutants, including E coli, so I'd say if someone got sick drinking the water, they either didn't filter it or they need to replace the filters in the bottle they used.
As has been said, many Fed. lands have been grazed for many many years. Often, the lands are on a rotation, where they are grazed every third year but that may not be true state wide. Seems like it would make more since to find a new location where no grazing is allowed, or where grazing isn't happening at the same time as you plan to hunt. You're not going to stop ranchers from leasing the lands for grazing by not coming to Colo. to hunt, that much is for sure. If it were me, I'd not give up something I love to do just because of cattle.
I'd be interested to hear from the Ntional Forest service on the legality of a "rancher building a cabin and barn" on National Forest property, with road access past an obviously posted "no motorized vehicle" sign.