You already know that pristine-looking backcountry lakes and streams can be contaminated with tiny, gut- twisting bacteria, protozoa, and viruses. But do you know which of the many water treatment methods on the market is the best for you? Here's how to decide.

You want One quick method for use domestically and abroad

Choose An ultraviolet light purifier like the SteriPEN ($80-$100, steripen.com) bombards water with UV rays, neutralizing bacteria, protozoa, and viruses (which are a particular problem in developing countries). It works in less than a minute and doesn't leave a chemical aftertaste. Caution: This handheld device works best in relatively clear water (strain with a bandana first if it's not) and requires batteries (pack spares).

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You want The lightest possible treatment

Choose Chlorine dioxide tablets such as Aquamira ($8, aquamira .com) weigh almost nothing (less than an ounce for 30) and take care of all three major types of bugs with a highly active form of oxygen. They're also easy to use—just pop one tablet into your bottle—but they take 30 minutes to kill Giardia and up to four hours for cryptosporidium. (Iodine tablets are also lightweight and cheap, but they won't kill crypto and leave an unpleasant taste.)

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You want The best method for silty water

Choose Good old-fashioned boiling works everywhere, but it's perfect for ultracloudy rivers and sediment-choked puddles. Collect water in a pot, then fire up the stove: Not only will the high temperature kill bacteria, protozoa, and viruses, but the boiling will help the sediment settle out, leaving clear water on top. Simply bringing water to a rolling boil is sufficient. Cons: Boiling is fuel-intensive and requires waiting for water to cool.

You want An easy, speedy method

Choose Pump filters use microscopic pores (.2 microns or less) to snag bacteria and protozoa while allowing water to flow through the filter at one to three liters per minute. They shine where viruses aren't an issue, but can be pricy and require field maintenance. Where viruses are a problem (developing countries, near human habitation or agricultural runoff), choose a filter with iodine resin, upgrade to a purifier (First Need's XL model neutralizes the big three pathogens), or back up with a chemical treatment.

You want A hassle-free method for big groups

Choose Gravity filters are quick and trap everything a pump model does, but handle larger volumes of water. (The Platypus CleanStream filters four liters in less than three minutes; $90, platy.com). The lightweight bags pack small and are a snap to use (just hang from a tree, let water drip, and drink).