I've had what I believe to be Marek's Disease affect birds as young as a couple months old, but none this age. They presented initially with leg weakness, then with seizure-like activity. After that I culled them.
My personal experience with chicks that present like this one, is that they don't usually recover. Some live through it, but remain weaklings and die later at a young age. I tend to think it is usually an intestinal bug of some type. Does the chick in question have normal stools?
I suspect Tim63 is right. If you don't think the chick is suffering, you can separate it, give it food and water, keep it warm, and let nature take it's course. If you have any inkling that it is suffering, the kindest thing to do is to put it down swiftly, painful as that is to you.