Actually, most of the bad guys will die if you boil the water at any altitude, including the summit of Everest, where the boiling point is around 165 degrees F. the temperature at which milk is pasteurized and at which cooked meat is considered to be "safely cooked" ( my understanding, at least). Just bringing the water to a good rolling boil should be sufficient at any altitude. I believe many people are being excessively cautious by prolonging the boiling time for disinfecting purposes.