I don't think most new chicken owners think about it either.
I think everyone should learn about how serious chicken disease are and how easily they are spread before they get chickens but how in the world would a non chicken owner ever know there is such a think to learn about?
Not sure if that came out right but hopefully it makes some sense.
I also don't think many understand that is is just wrong (in my eyes) to sell sick birds yet they continue to do so.
Close your flock and try to breed for resistance.