Some "experts" have not kept up with where the problems are. Over the years , commercial producers have been using practice measures to control Marek's, like all-in-all-out, vaccination, disinfecting the premises between hatches. Backyarders have not. Now our problem is growing mainly because we have not been practicing those measures, and most chicken peole not knowing that there's a massive problem, not knowing that there a alot of symptoms other than one-foot-forward-one-back, thinking that this is the only symptom. Also , adding vaccinated birds that have no symptoms but are carriers.
From what I've read, most chickens contract Marek's from other chickens.; Large amounts of chickens not even a mile away , it would be less likely, as well as wild birds carrying it in. But not impossible.
Since all my chickens were hatched here or from a nursery EXCEPT FOR ONE, I have traced it back to that one. The closest chicken to her died with paralysis , grey eye, wasting, and lost control of neck. On the end he had the forward back legs. From then on I lost a chicken from wasting away every 3-4 months (while eating and acting normal).
Don't discount your flock history.