The problem is with the breeders that baby these birds, treat them, and then breed them. There's a big difference between a little preventative care, and bringing a bird back from near death from some disease, then breeding it because it had the perfect tail shape. If they still get sick even after the preventative care, then they are weak and only perpetuating a weak genetic line. This is why I feel so conflicted when I go to poultry shows, as I see so many mutated and sickly birds winning awards, but I still go.. It's like morbid curiosity.
Then, the parent bird(s) and all those offspring should be culled. How many generations would it take to breed out that defective gene, if possible? Also, who is carrying that cootie...the male or female? Maybe swap breeding partners and see what the next generation brings.