Here is where necropsy could have gone so far in helping you figure out what your problem is. If it was Mareks it would have shown up then. If you lose another one take it to your vet and ask them to send it to the state for necropsy.
There are a lot of things it could be. Have you had a fecal done? It could be parasite related. It could be contaminated feed, either straight from the bag or any that is laying on the floor that they are picking at. Depending on the bedding you use it could also be that. Are you using corn? That can also cause death if its gotten damp.