Just some thoughts on Marek's. Last summer I lost several hens, never knew what it was. Then a neighbor who also keeps chickens told me she lost several also and the vet diagnosed it as Marek's, said there was quite a bit of it going around. The symptoms matched what I saw in my flock, so I assumed Marek's was the likely cause.
Did some research and found out Marek's is cause by the herpes simplex virus, same one as causes cold sores and the like in humans. I recalled from the 1970's that some people used BHT as a remedy for herpes in humans (used to be able to buy the stuff at health food stores, now online mostly). So I started adding some BHT to the chicken food for my two remaining birds who were having some symptoms. Well they got better, and went from having balance/walking problems and feather loss and no laying, to able to walk and run, regrew their feathers, and are now laying normally (for 3yr old birds).
I can't say that it was the BHT, nor do I know that it was Marek's which was causing the problems, but the results were very good. Given that BHT has a history of working on this virus in humans, and is relatively cheap, I thought I would pass along my experience. FWIW, the dose I used was roughly (very roughly) 20mg per bird per day, mixed with their treat (sprouted grain).
BTW, There is nothing which will eliminate the virus once it is in an animal (human, bird, whatever). At best BHT or prescription meds will halt the virus and allow the body to do its cleanup, but the virus always remains, dormant until it gets an opportunity for another outbreak (in whatever animal host). So if BHT works it is not going to make your birds virus free nor eliminate the virus in your yard. But it may be a very inexpensive way of saving exposed birds and keeping the disease at bay.