Okay it's 2019 now but Google brought me this page so maybe it's the most relevant.
Anyway, I did a degree in Biochemistry and a virus cannot replicate itself. What it does is inject its RNA (like DNA) into your cell, which zombies the cell into making nothing but copies of the virus until the cell bursts, releasing more into the bloodstream to infect other cells. Of course we would die in a matter of days without help but when the strange molecules of the virus trigger an alert and your white blood cells go crazy trying on shoes to fit the virus until a white cell is found that fits. That white cell then multiplies at a rapid pace and pretty soon you have lots of clumps of white blood cells mopping up the viruses.
So....
By the time you feel off, those little buggers are already in your bloodstream.
This is the part that gets me: why do they all travel back to the nasal cavity, climb out onto the surface of your skin and take a tour there, only to get zapped by First Defence?
OR is this perhaps the case: when you get an infection, there are thousands of the little buggers sniffed into the back of your nose, and only a few can gain entrance into your body at a time... so a few get in and cause mayhem, but before the whole army gets in, First Defence captures the lazy or cowardly ones at the back? Leaving far less inside your body for your white blood cells to deal with. I guess that might be... still seems a bit odd though.