I bought a couple of nano masks a while back for just such an occasion (well, for the bird flu actually). It filters out 10 nm sized objects. Typical N95 masks seem to be limited to 300 nm sized objects: “Thus, it should be emphasized that the certified N95 respirators will
protect their wearers properly against the particles of
300 nm and larger, but their performance may be below
the threshold for aerosol particles of the nanosize
range. The penetration values of the nanoparticles
through N95 respirators depend on their filter media
characteristics”
http://www.medtau.org/ice-ccm/pandemic/N95.pdf
“Examples of viruses that can pass through current filtration devices but not nanofilters are: Hepatitis B, HIV, Ebola, Hanta, Influenza A/B/C, and SARS”.
http://www.brianlaks.com/nanofilters.htm
A flu virus is around 80 nm in diameter btw.
Some day the nano masks may take over, but in the interim the N95 is considered to do an adequate job. Although some bare virus particles stripped of their mucous-y coatings may get through, probably not many do and you need a critical mass to get an infection. Also, the limited 48 hour life of the nano medium outside the sealed packaging is a consideration you need not factor in with the N95s.
The virus must remain attached to a moisture source such as a sputum droplet. When the droplets hit the masks, the masks stop the droplets and they dry out. The masks are not designed to stop the movement of a virus-sized particle.