-- Last Updated: Dec-30-12 10:21 AM EST --
Has much to do with its cleanliness. High stream with little chance of mammals, and a spring at the source, probably ok. Low land lake with human waste present, higher chance. Filtering and boiling seems a little unnecessary for most water sources, unless you are worried about viruses, but running through a filter isn't a huge hassle. A uv treatment couldnt hurt either. As with most food and water diseases, cross contamination is your real worry.