How old are your chicks? Are they in a brooder or outside? Have they been vaccinated for Mareks or anything else? Is your sick chick showing any other signs of illness? There are some BYC members who are very experienced with Mareks, but from what I've read, it doesn't normally show up in a chick until they are 8-10 weeks old. I don't think it shows up at all in chicks under 3 weeks, but don't quote me on that. Have you recently put your chicks outside? Do you think it could be coccidiosis?
Chicks are susceptible to a number of illnesses, and there are vitamin deficiencies that can cause the leg paralysis. I'm not much help because I'm dealing with this issue right now with one of my 3 week old chicks. My chick stopped eating and drinking entirely, and she had very labored breathing for a day or so. I gave her a drop of Poly-Vi-Sol (without iron) twice a day in case it's a vitamin deficiency, and I tried to give her a few drops of watered down hb egg yolk several times a day. She was like that for two days, then day three she perked up a little bit and will drink if I hold her to the water dish, and she started eating. She's still very weak and can't stand, sort of flops around, but I'm hoping she will get stronger as she continues to eat.
Sorry I'm not more help, but if your chick is getting picked on or not eating, I'd separate her, give her Poly-Vi-Sol or electrolytes in her water, and just watch her for a few days. If you think it might be coccidiosis, I'd treat your entire flock with Corid. If you think it's Mareks, there are other BYC members who will be much more help than me, but I hope it's not Mareks. Good luck.