Well Marek's does usually affect chicks with symptoms, simply because most chickens have been exposed to marek's by the time they make it to adult hood. If by chance you have a magical place that marek's wasn't for a long time, but then it was it might still cause an issue. But it doesn't usually affect birds over the age of 24 weeks. Except it can cause adult birds to drop dead for no reason, or rather when people have a bird die for no reason they get it tested, and of course it is positive for marek's...but is that what actually killed it? Who knows...anyway, what happened with my chicks....One days I look out and a chick is limping, so I go check...nothing wrong with it, doesn't appear to be in any sort of pain, is still mobile, doesn't really like me getting near it though. 2 weeks go by, chicken is still just limping around, but having a few difficulties, he was trying to cross the ramp to come back in the coop, and he got knocked off and landed in a big mudpuddle, and was just sitting there all wet and cold, anyway I went down and got him and cleaned him up, and from then on he followed me everywhere, and would sit in my lap, and just loved to be near me. Anyway he starts getting really mobile on that leg and doing really well, and the very same day that he starts doing a great, one of the pullets starts limping just the same way...with the foot curled. Weeks go by with 2 limping birds, nobody getting better, nobody getting worse, poops were good, eating good, moving around, limpy but good, drinking good..Then the pullet got to where she was limping on both legs and using her hocks all the time...then she couldn't move around at all...she was scissor legged, one out front and one behind. She was still eating and drinking with clear eyes, and seemingly normal, but confused about her own lack of mobility, and I just couldn't bear to watch her suffer anymore. I tried vitamins and what have you, but nothing helped, So roughly a month after the first symptom we had to cull the pullet, and then I went up 2 days later and my sweet little cockerel couldn't move his head. It was dropped down under him...Awful, so we had to cull him to. And now everyone is fine and doing well, young and old alike. So whatever was going to happen, has happened I feel like. This very well might not be your issue, but I wouldn't say it is impossible...it would go against the grain a little though with what I have read...but it seems to be so darn far reaching.