SPOILER: If everyone are already infected, then why do they start transforming after they get bitten?

I'm rewatching the series and I just stumbled across what the scientist told nick about the virus.

I don't really get it? How does a bite or a scratch triggers the fever if they're already infected? It doesn't really makes sense to me. Also throughout the series we see the characters get in contact with walkers blood all the time, so what makes it different if you get bitten or scratched?

Submitted July 30, 2021 at 08:20AM by Evening_Average https://ift.tt/3xfMETM