I could be totally wrong. And its not the best thing to admit but i think i have Mareks in my flock. My Jersey Giants were wiped out. along with birds a year later suddenly going lame and die in same time frames. Why i think i have it in my flock: most chickens that are mature have a "red ring" in their eye color.... i know as chicks and juveniles their eye color some birds have gone lame in one leg, sometimes both.... some die and some recover. some have survived to have a slight limp, they now "gallop" Here is the concept: because the vaccine is only good for chicks... what if we just let our flock .... evolve to become resistant. Some breeds never got sick, just got the eye color.... In one rooster i am not sure if its the disease or genetic but his one eye went blind/broke. it dialated and never went back. blind cos i can sneak up on him on the right. I cannot in my heart kill any of the birds just because they are carriers... Its like killing me just because i have mono latent in my immune system. as an example here is my little roo's good eye (other side is blind) his original pupil color was more gold.... now it is saturated red. some of my birds have had red pigment develop and some never were affected... and some have the pure black eyes...... he has a brother whos eyes just have the red still limited as a ring. this guy is just my best example.