Waterborne viruses in North America would likely be the result of sewage contamination, which in the backcountry would mean really appalling toilet habits from your fellow backcountry travelers--something that's thankfully rare. In the West I concern myself with cysts and bacteria, which the Sawyer Squeeze handles easily. For virus concerns I like UV.

FWIW Sawyer makes a "point zero one" filter cartridge that removes viruses. Its flow rate is supposedly about half of the standard cartridge.

Cheers,
_________________________
--Rick