Just joined!

Hello...I'm a newbie! New to chickens and a new member. I have joined because I have a chicken problem and believe that I may have a six month old speckled sussex with Mareks. She is part of a group of five that were vaccinated and have been part of our small flock since July. She has been symptomatic for about two weeks but there are no signs that anyone else is infected. Her symptoms started as a slight limp, which progressed to a major limp after about three days. I kept her in a small coop outside with her mates, but let the mates out during the day. She eats and drinks as per normal. Her limp has now progressed to her tripping over her wing and getting stuck on her side periodically. There are no visible lumps or bumps, no discoloration, nothing that seems broken. It is confusing and reading through all the info is not making it less confusing!
At this point I need to know is it worth stressing her out by completely separating her from the flock? I know if it is Mareks the entire yard and flock will already have been contaminated. Should I just euthanize her and get her tested? Not sure what the best course of action is and can only hope that I have not thoroughly contaminated my flock and all ideas of starting my small hatchery. Any advice is appreciated.
Cheers!