Waterborne pathogenic viruses range in size from 0.020-0.030 um, and are too small to be filtered out by a mechanical filter. All waterborne enteric viruses affecting humans occur solely in humans, thus animal waste doesn't present much of a viral threat. At the present viruses don't present a major hazard to people drinking surface water in the US, but this could change in a survival situation as the level of human sanitation is reduced. Viruses do tend to show up even in remote areas, so case can be made for eliminating them now. Physical Treatment Heat Treatment Boiling is one guaranteed way to purify water of all pathogens.
Most experts feel that if the water reaches a rolling boil it is safe.
If one wishes to do this, a pressure cooker would allow the water to be kept at boiling with out loosing the heat to evaporation. African aid agencies figure it takes 1 kg of wood to boil 1 liter of water. Hardwoods and efficient stoves would improve on this. Water can also be treated at below boiling temperatures, if contact time is increased. Heat exchangers recover most of the energy used to warm the water.