"No filter makes sense unless it also removes viruses, because you still have to chemically treat it, or boil it to kill them. So why not just do that in the first place?
The Sock"

IMO, pending more information, viruses aren't really much of a problem as long as humans aren't present. Of course finding an area not traveled by humans is another issue entirely. The wilderness is getting smaller every day and what wilderness there is is increasingly less wild than it used to be.

That said even when humans are doing their thing in the water it is fairly rare for anyone to get infected by otherwise well filtered water. The threat is far more theoretical than actual IMO. People infected usually have been exposed to water that was not filtered. The reason being that viruses are pretty fragile out on their own and so they tend to travel contained in or attached to other materials that are easily removed from water by even a basic filter.

This is related to the old argument over using condoms. Claim is that the pores in a latex condom are larger than the size of an AIDS virus so condoms are useless. But studies of those who use condoms consistently show that condoms stops the transmission of AIDS.

Reason being that the virus doesn't travel out on its own. it attaches to other materials and rides. Stop the transmission of these much larger biological materials and you stop the virus.