While viruses are not commonly encountered in the great outdoors there is no doubt that many areas of the world that we travel, or live within, that are contaminated with viruses (hepatitis A, norovirus, rotavirus, enterovirus, etc.)
In the world of water filtration, there is the need for ever smaller micron levels of filtration in order to prevent viruses from passing through the filter.
What is the required micron level in order to filter out all viruses from contaminated water?
And because there are multiple standards out there, please reference which standard you are basing your data/answer on.
Additionally, because it is a rather important part of micron filtering, be sure to include if the micron level must be absolute or nominal.