You do not need to worry about viruses though when purifying water apparently. I do not believe them. Hepatitis comes from shellfish that are improperly cooked. Doing your business 100m away may help, but in some places its hard to get 100m away and iff you do, whoes to say that layer of clay will not just shunt it straight into the water course. And thats before getting to the major slow flowing rivers.
Agreed on most points. That said, if you can't get 100 yards away, then you're probably near enough to civilization to wait to use a proper facility.