I found this APA link_ http://apa-abayouthprogramsite.org/educational_material.htm I am pretty inpressed with their information. My flock dealt with a mini-Mareks outbreak. We lost 3 birds from our mostly immunized flock. What I didn't know and I just read was that birds develop a natural immunity to Mareks by 5 mo. of age. If I had known that I would have kept my non-immunized birds seperate.