I posted this message under another topic when discussing vaccines. I will copy and paste it here:
Actually Marek's vaccine is recommended for small flocks. At least, that is what I have been told by vets specializing in poultry. The few sources I have found that don't recommend it give the reasoning that it is cost prohibitive because it can only be bought in doses of 1000 or more. They assume that spending $13 for 1000 doses is stupid for 10 chicks because $12.87 worth of it will be thrown away. Yes, a bird can become a carrier for life, but virtually all birds carry Mareks latently in their system. Mareks is worldwide. If your birds have ever free ranged or been exposed to an area that ever had a free range bird in it, it has been exposed. If your bird has ever been anyplace that a wild bird has been, it has probably been exposed. If you have been in an area with wild birds you have probably brought the virus in on your clothes. Last year I had a bird come down with Mareks. A poultry vet informed me that it is everywhere in the environment. Virtually all birds are exposed. The reason it is recommended that day old chicks be vaccinated is because at that point they probably have not been exposed to it. If you never experience Mareks, you are lucky. I personally will never not have a bird vaccinated for it.
Since my Mildred came down with it last year (and lived) I have studied up on it quite a bit. Be wary of internet sights written by people that are not in the Veterinary or science field. There is a lot of bogus information out there. My sources have been vets in poultry departments of universities, Fort Dodge (they make the vaccine), etc. Vaccinating is so cheap, why not have it?
Also, the cocci vaccine is the one that they are referring to when talking about medicated feed, not Mareks. And according to the vets at Fort Dodge (they make Mareks vaccine), if you have a chick that is a few days old, it can still be successfully vaccinated as long as it has not been exposed to other poultry that may have been previously exposed.