Yeah, I thought that too originally, but I've also read otherwise. Here's the web address
http://www.firststatevetsupply.com/poultry-health/mareks-disease.html
I'm so glad you asked that question! I was thinking of the same thing, the only problem is my chicks aren't vaccinated for Marek's yet. I am waiting for it to come from Jeffer's. Would it be safe to let them out in the yard without a vaccination?