M2H: Forgive this graphic question. If you have a chance to speak with the vet again, I would ask what preliminary results lead her to believe it was Marek's. I would press her if she actually visualized tumors anywhere. Eliza's sudden limp might be indicative of a tumor pressing on a nerve--potential Mareks'.
My understanding: Marek's is in our environment from wild birds, etc., and our flocks can build an immunity to it naturally or via vaccination. Either it kills them when they're young, they build immunity, or tumors grow slowly inside them (quasi-immunity) the vaccine is not 100% successful.
I had my chicks vaccinated for Marek's and still lost one at about 4 months: sudden paralysis and eventual euthanasia. Unknown reason for paralysis, could be injury or could be Marek's. Any Marek's blood test would come back positive because she was vaccinated as a chick, so that was not a helpful test.
Even though I had a pullet suffering an unusual sudden paralysis which could be indicative of Marek's symptoms, we've since broody hatched 4 clutches of chicks and not vaccinated the last 3 against Marek's. No chick deaths.
I'm a layperson. My post is my personal experience, nothing more.