Mareks can live for several years in the environment. Once you have it, you have to assume all your hens have it and are shedding the virus through dander and feathers. There is a vaccine for it, but it doesn't prevent the spread of Marek's, just the growth of tumors.
I am not allowed to have a rooster, so I can't breed my own immune flock. The best I can do is buy chicks from breeds that aren't as susceptible, although it varies by individual.
I haven't decided if I want to buy vaccinated chicks. I may try just getting unvaccinated chicks. The strain of Marek's only affected 1 of my hens, so it isn't one of the more virulent ones. Once I vaccinate, all will have to be vaccinated.
From what I've found online, here are breeds that are most and least susceptible:
Most immune / least susceptible
Campine? (Supposedly descended from the fayoumi so maybe)
*certain bloodlines. Things I read said leghorn were less susceptible than rode island reds