Excellent point. I was thinking of the back country/wilderness situation.

I do believe boiling is the way to go if you want to be sure. It is interesting that there are various recommendations for the length of time required to boil water to insure sterilization. I have seen recommendations for up to ten minutes, with five minutes often given as a requirement.

For many knowledgeable authorities, these times are excessively long. It is necessary only to bring the water to a rolling boil in order to kill pathogens.

The five minute rule apparently derives from the special case where you are sterilizing cloth in order to prepare dressings. There is evidently some kind of nasty beastie involved which does indeed require that much exposure to boiling conditions in order to die - can't remember the specifics for the life of me..