Thanks, @jeria. Marek's is one of those things that you always think 'Gee I hope I never get that in my flock!' Then one day you realize it's what's been killing off your birds and you think 'Crap! How did this get in my flock?'
I'm still trying to figure out that one. We have a lot of Amish around us, this past year we have also been having a lot of problems in our are with foxes. My bantam crosses came from eggs that I bought from one of our Amish neighbors and so far they have fared better than the Standard sized birds have. I have lost three of 43 bantam crosses to Marek's and over 15 Buff Orpingtons and Welsummers. The die off from the initial infection slowed and then stopped by last July. I have three Buff Os with ocular Marek's but holding their own. It's been very heartbreaking.
I managed to talk to one of the vets down at Mizzou's veterinary science lab and he told me the moment that I told him about my losses that it sure sounded like Marek's to him even without a necropsy. After that I noticed the three birds with grey eye and asked my husband who is a retired eye doctor if he would look in their eyes and give me a diagnosis. Bless, him, he's a trooper. He took his portable slit lamp microscope out to the coop and looked in the bird's eyes for me. Sure enough. He said one word. Herpes.
I really suggest that everyone familiarize themselves with the disease. It's everywhere and no flock is 100% immune, even vaccinated birds can succumb and there is no cure.
I did everything right. Bought chicks from an NPIP breeder, practiced good biosecurity but all it takes is one wild fox dragging it's kill in the form of a neighbor's Guinea fowl or hen through the property and all bets are off.