Consensus seems to be that viruses are not an issue in the States.
Not sure why, but does not seem to be an issue.
I have been using the Sawyer filter for about 3 years now as my only water treatment on my hikes in the SF Bay Area and the Sierras.
Never had an issue, never been sick, still using the same ole filter with no problems.
If you are concerned about that, you could add chemicals to the water after treating or you might be better off with a Steripen.
I like the Sawyer because I psychologically don't like the idea of particles and chunky things in the water I drink and the Sawyer takes care of that for me and is faster than chemicals.
Nothing wrong with the Steripen at all....have a friend who used it for 9 days on the Tahoe Rim Trail with no problem, so I know it works too.