I amjust now reading a source, the 2010 edition of Medicine for Mountaineering, which states, "The First Need filter has an absolute pore size of 0.4 microns; the Sawyer Viral Purifier has an absolute pore size of 0.2 microns. Both have been demonstrated to remove viruses as well as bacteria and parasites in studies conducted for the EPA." (p.66) At least in the USA, I understand virus removal is not an issue in the backcountry.

This book has a very thorough discussion of water disinfection.