Mareks hardly ever strikes a chick less than 3 weeks old, and it is usually older when it occurs. I would suspect dehydration or a vitamin deficiency, but Avian Encephalomyelitis (AE) is one thing to rule out. Take this chick, and keep it warm and dip it's beak into water with electrolytes and vitamins several times every hour today to get it re-hydrated. Where did you get the chicks? I hope it starts to improve with fluids. Here is some info on AE:
https://en.wikivet.net/Avian_Encephalomyelitis
http://www.thepoultrysite.com/diseaseinfo/9/avian-encephalomyelitis-epidemic-tremors/
https://en.wikivet.net/Avian_Encephalomyelitis
http://www.thepoultrysite.com/diseaseinfo/9/avian-encephalomyelitis-epidemic-tremors/