I had a similar situation recently. I was sure the bird would die but it didn't! I kept it in the warmth and made sure it had its favorite treats to encourage her to eat. I wouldn't doubt that I have Mareks in the flock but they must be resistant. Chickens can have so many things wrong with them and we never know exactly what it is.
Good luck and hope yours survives too.
Good luck and hope yours survives too.