SteriPEN Ultralight UV Purifier 

This is actually complementary to a Sawyer filter. A Sawyer filter (or similar technology) mechanically filters everything in the water which is larger than 0,1 micron, but viruses are smaller. If you want both a clean water and a water which is not just free from bacteria and protozoa, but also viruses, then this thing is useful (and fast), or a pill (equally effective but you have to wait 30 minutes, and you drink the chlorine). I think I am buying one of those to treat the water after the filtering with the Sawyer. It's a little expense and weight more, but in that way the water is completely safe, however ugly it seemed at first.