Thanks you a lot i read somwhere that one that survive Will ne imune to that desise and also can carry that imunity throw gens to their offspringsSome 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.