My grand child noticed my bad breath, so before I had a dentist appointment I bought dental varnish and applied it. Not been sick for a couple of years, I caught a viral infection within 24 hours after covering up the cavity (and other teeth). Coincidence? Can bacteria fight viruses?
Only way I can see it helping is by driving people away from you before they can expose you to their airborne viruses. :dubious: And even that wouldn’t be very effective, because they can probably inoculate you before they can smell your breath. I really really hope. 
ETA: This is GQ, so I’m not joking. I’m no doctor, but I know of no medical mechanism that would reduce susceptibility to viral illnesses except by reducing exposure – by keeping people away from you.
So, yeah, probably coincidence. (Or exposure to a dental professional with an undetected contagious case of a viral illness.)
I am a doctor, and I concur, the only mechanism I see is as gnoitall (presumably derived from the word ‘Nom’, meaning “wise” in the tongue of the early Edain. Or perhaps not) describes, making virus-laden people avoid you.
More directly, viruses have no sense of smell.
I didn’t address it earlier, but OP has a suggested alternate mechanism:
I’ve never heard of bacteria impeding viral infection in any fashion. Unlike certain fungi which can impede bacterial infection (i.e., antibiotics, but I’ve also never heard of Pennicilium fungus infection either, so that’s also completely hypothetical.)
As far as I know, bacteria and viruses co-exist just fine, unless the virus is a specific bacteriophage, in which case it would have no interest in making you sick at all.
You probably caught it from tht grandchild, who was close enough to smell your breath.
Young children are a common vector for such minor infections – they get them from other children in the classroom, and bring them home to their relatives.
I should have added that most of my mouth bacteria (good or bad) must have been killed, because this stuff is very heavy
https://www.google.fi/url?sa=t&source=web&rct=j&url=http://www.prevestdenpro.com/prevestadmin/files/2017410135955435.pdf&ved=0ahUKEwih9ePvy4HXAhWqKJoKHQ8MA-AQFghMMAo&usg=AOvVaw0YMmZDshY0bqK_UBrNFTFR
My mouth was numb for at least an hour afterwards.
[George Carlin]Silly, germs have bad breath[/George Carlin]