@Airyaman I am so sorry you are going through this. Were these chicks that you hatched on your own or did they come from somewhere else? Personally I would probably cull any in that flock that are having problems that diminish their quality of life (easier said than done, I know!)
There is something that can help with Mareks, I believe it is a skullcap mushroom supplement. I think I have seen
@Debbie292d recommend it on here, maybe she could give more info. I think it is something that you mix in with their food to prevent symptoms or spread. I’m sorry my mind is totally spacing out right now.