If someone has Marek's exposed birds, the best thing to do is have some common meds. Like Corrid, or Sulfadimethoxine, Tylan, Penicillin, etc. Or if vaccinating for Marek's , vaccinate for Coccidiosis. I've lost birds from opportunistic bugs. I also lost 3 to Necrotic Enteritis , no symptoms the day before. You're right, their immune system is shot to heck even being vaccinated.