Yes it is contagious, but if you have one with it, they have probably all been exposed at the same time. Just remember that many things can appear to be Mareks, and it is best to get it confirmed with a necropsy after the chicken dies, hopefully a long time from now. Vitamin or mineral deficiency, infections, and other things may cause symptoms. I have a hen who had been normal for 6 years, then suddenly could not keep her balance. She stayed like that for over a month, but now walks all around, albeit a little wobbly. I would just give her special care to make sure that she can reach her food and water, protect her from being hurt or bullied, If you lose her, keep her body cold (not frozen,) and contact your state vet for a necropsy.