Drinking straight from crystalline mountain streams is nice in principle, but a lot less awesome when we end up running for the bushes every five minutes for the rest of the trip. There are plenty of ways to prevent waterborne bummers, but none that we find as effective, easy to use, and reliable as the MSR Guardian Purifier System. Developed for, and in conjunction with, the US military, the Guardian uses new Hollow Fiber technology to filter out just about everything (protozoa, bacteria, particulate, and even viruses), so you can sip fresh, clean water without fear. The Guardian has an easy pump-action design that can filter up to 2. 5 liters per minute, and an ingenious self-cleaning design--10% of the water from every stroke flushes contaminants from the filter, so it never gets clogged or becomes less effective.