Thanks you a lotSome are affected, some never will be. It's really just up to each individual bird on how they're affected. If they were vaccinated, they could carry it and never show a symptom. Or they could show symptoms but never get bad.
I wouldn't cull an entire flock unless you have proof on paper that they are positive for it. Even then, I personally would only cull when necessary. Just make sure that you keep a closed flock. No birds leave.

