I understand your question, OP.
For most CFer's, a virus can aggravate the bacteria we are currently colonized with. Therefore a viral issue can quickly turn into a viral and bacterial issue.
When I was younger, my PA didn't flare up when i got a cold. I could kick the cold and that would be the end of the story.
But the past 10 years or so, every time I get a cold my PA flares up and I need abx.
The general recommendation is that if you don't feel better in 10 days from a cold, the virus is generally gone by 10 days so it's considered bacteria.
Since I know myself, I no longer wait 10 days to see if my issue is bacterial. I know what my PA flare up feels like, so I generally wait 2-3 days before getting on abx. I have 3-4 strains of PA and one of them is bound to flourish when I catch a virus.
And there's no sense in waiting 10 days for lung damage to occur - for me it's a clear pattern with viruses so I aggressively treat with abx.
Good question!