Boiling: Handy to know the answer if you are out backpacking or taking a trip to the less civilized parts of the world. Common options are to use chlorine, a ceramic filter, iodine tablets or to boil the water to treat it. Boiling water is the only known method that is 100% effective in destroying pathogens known to cause sickness. The Centers for Disease Control have a very handy chart comparing the effectiveness of various water treatments. It may not be as convenient as the other methods but it works and is effective.