Ambient conditions are important, for sure.
But so is the level of innoculation. They're all important, because what we're really talking about here is the equilibrium of a biological system. Your plants might enjoy an equilibrium point such that a major influx of certain pathogens wouldn't harm them at all. Such an influx would change the equilibrium point of your plants, but not far enough for the pathogens to take over. Eventually the equilibrium point would move back to where it started.