Trisha's counsel was sound advice and spot on. I would only add that a Google search will quickly turn up oodles of info on Mareks and it would be well worth your while to read a bit up on it. The best info, as I recall, is on the Merck's Vet website.
As for vaccinating or not, I made a decision a few years ago to stop vaccinating and to stop medicating my birds altogether. There was a time when I just about gave up on the Barnies because the majority of them were constantly getting sick. Not so much Mareks but moreso with CRD. Finally, I culled down to one male and two females that never got so much as a sneeze or sniffle. I've rebuilt from there and to date I rarely get any sick birds at all. Given the weather here, from 70-75 one day down to 40-45 the next every other day or so, I can tell you that I've noticed a significant improvement.
When I was vaccinating and medicating, I found myself constantly caring for birds. Just about the time the birds would get clear-eyed again and not have a runny nose anymore, the temp would dip and I'd be back at it again. Or everything would clear up for a few months and then as soon as the temp started to drop and the precipitation started to come again, so would the runny noses and bubbly eyes.
So, my advice is to selectively cull for those sick birds and breed only the healthiest. Along the same lines, I recommend following the advice of some other "Old Timers" who say to breed only 2-year old birds or older as breeding pullets does not give you enough time to tell how disease resistant they are.
God Bless,
