I bought three birds, two pullets and a grown rooster--all Ameraucana, a rarity in these parts--from a dealer five hours' drive away. They were to be the start of my flock. Before the pullets had lain a single egg, they passed, and the rooster died of blindness on a hot day when he could not find the water. The symptoms for all three were consistent with Marek's, but I never got a confirmed diagnosis.
From that day to this, I have not wanted to buy birds. Only the eggs--and incubate them myself. Lesson learned.
I have incubated eggs and still have a few chickens in the coop--young ones. The ones I have are now about four months old, and no sign (yet) of Marek's in them. But I don't expect to be out of the woods yet, based on what everyone says about Marek's. So I have some new chicks kept inside the house, waiting to be transferred in a week or two to a new property which has not had chickens yet, and I'm doing everything I reasonably can do to keep them isolated from those outdoors which have been exposed. I've bleached the floors, feeding pans, light fixture, etc. in the brooder and have no intention of allowing any contact with the chickens outside.
Who knows? Maybe the chickens that I think have been exposed will be fine. So far, they have shown no symptoms. Just the same, I no longer wish to buy birds. I'd rather buy the eggs, incubate them myself, and never have a bird of my flock that has contacted chickens outside my flock--since, as I understand, Marek's and many other diseases are not transmitted via the egg.
Furthermore, having researched the vaccine issue (Marek's vaccine is called a "leaky vaccine" because it does not confer immunity, and it only prevents the bird from dying of the symptoms while allowing it to spread the virus to others en masse), I will NOT vaccinate any of my flock. The best is simply to keep them from contacting possibly infected individuals.