In reply to Snoweider:
It's a massive subject, and as you say, with few independent studies. For manufacturers it is understandable that they will focus on what their products definitely can do though.
If I'm travelling to somewhere that I know that viruses will be likely then I will carry chemical purification. Even the very fine filters you mention do not remove all viruses, and it doesn't take much to make you ill. They are also only as good as your own personal hygiene and how good you are at keeping the 'clean' end clean... The viral outbreaks you see on the AT amongst dozens of experienced hikers using good filters show that there are no guarantees.
Steripens seem to be another reliable option, so long as you can get the sediment out of the water, but that goes for chemical purification too.
Water quality in remote areas is a weird one - I quite happily drank from streams all over the uplands of the UK until we ran a course for staff from Dwr Cymru who told me the results of some random tests they did in Snowdonia. From then on I filtered/sterilised/boiled everything if I had the choice.
Agreed on the motley selection of water filtration/purification systems we see. The guy that turned up with a kilo of cayenne powder is still the winner...