Even pristine looking waterways in remote areas can harbor contaminants.Even if you haven't been there, you've probably seen pictures of isolated mountain streams and lakes that look untouched by the hand of man. What could possibly be dangerous about drinking water so obviously free of pollution? Actually, quite a lot. Man-made contaminants are certainly a concern and are the main reason that untreated water is not safe to drink in urban survival scenarios. However, natural contaminants do exist and could make drinking water that appears to be clean a fatal mistake. The biggest (smallest?) offenders are protozoa. If you had a high school biology class you might remember watching these little guys through a microscope as they swam around in a drop of scummy pond water. The fact is they can live in clear water too. There are a number of kinds of protozoa, but there are several in particular that do nasty things to people. Giardia and Cryptosporidium can both infect humans resulting in abdominal cramping, nausea and diarrhea. Left untreated, infection from either can lead to fatal dehydration. Fortunately, neither of these creatures can survive high temperatures. So if you're in a remote environment and happen upon a pristine looking water source, feel free to drink. Just make sure to boil the water for at least three minutes first.