Mareks is one of those things that can be carried in by the wind and expose the whole flock. Mycoplasma based respiratory diseases are the same way. If one bird shows symptoms you can be assure the whole flock was exposed. In theory the ones that don't show symptoms any time soon after the first one did are resistant. The Mareks virus can complicate that though, because it can infect without external symptoms though by causing things like tumors in the reproductive organs instead. So once an outbreak happens its good to wait for awhile and see what happens.How would you know if they were resistant short of exposing them to something? or is it from breeding stock that the parents have already been exposed? lol, had chickens 3 years and still new to all this.
I'd love to know what you find as far as herbs to boost their immune systems.. I would definitely grow some of that..