Given the choice, chemically purified, filtered or boiled. Or all of the above. I've seen a dead beaver floating in a pond the morning after drinking water I'd taken from it. Glad I'd boiled that stuff. ICK!

I think a lot of this is from the ultralighters. I was glancing a book on lightening your trail hiking gear and they suggested it wasn't really needed, after all the author had been hiking for years without it and had never had a problem. *shudders*