Of course, you also have to deal with filter-proof organisms such as viruses. For that you need a purifying solution, such as MSR's SweetWater Purifier Solution ($9 per bottle). You add it to filtered water, let it stand for five minutes, then drink.
As a backup, there's old-fashioned Potable Aqua ($6; www.potableaqua.com), an iodine-based tablet that kills just about everything in the water from giardia on down the food chain. It doesn't taste great, but if you're in a hurry then it works fine. And no, you can't possibly drink enough of it for the iodine to pose the remotest danger.
Check out Outside Online's Filtration & Purification Buying Guide for more aqua-cleansing options.
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