As a boy scout, I regularly dipped my aluminum cup into mountain streams in eastern Oregon for a refreshing drink of cool water. No filter, other than what nature provided. Was I at risk? If so, I never had any ill effects.

Now under other circumstances, I wouldn't drink the water of a murky pond, or from an obviously contaminated source, but what about clear mountain water?

I carry purification tablets in my survival kit, but I wonder under what circumstances I'd use them.