Candice - you said something earlier that really stuck with me. Dis she hatch the chicks in an incubator or did she have a broody hatch them. Was the chicks at anytime with any of the older chickens? Of course, we all know they can get Marek's without even having contact with other chickens. The breeder said she hasn't had Marek's in a long time then it's telling me she still has Marek's, found this on shagbarkbantam.blog page:
Mareks is extremely contagious but does not spread vertically (to the egg). Youngsters should develop a natural immunity (called 'age resistance'), by the time they're five months old. This is one of the reasons it is important to raise your youngsters separately from your oldsters. The older birds that have encountered Mareks and have managed to survive are carriers. New birds coming in from other flocks are always potential carriers.
Mareks usually hits between 5 and 25 weeks of age, but can appear even later if the bird had 'latent' MD and is substantially stressed. However, if the bird is a few years old, I would suspect a similar disease called Lymphoid Leukosis (which does pass to the egg). Both diseases will produce internal lesions (or tumors), detectable upon post mortem examination, but LL does not produce paralysis.