I highly believe that one bird has Mareks...now what? Scared!!

One of my 5 month old black sex linked TSC babies was sluggish at least 10 days ago so I removed her from the flock. (Does TSC vaccinate their chicks?) She has been in a separate cage in the shed, away from the flock. She was eating and drinking the past week. Within five or so days she was falling over and not getting up. Her balance was off. Her neck was curving to the side. She would try to hold it up and it would go to the side. Her eyes were closed. She was breathing. After work this afternoon I went out to check on her and she was laying on her side. Her feet were stretched out. Eyes closed, I called to her and her head moved a bit, her eye opened then closed. She moved a bit but could not get up. I can't stand seeing her like this. I gave her water through a syringe. She wouldn't eat. I could call the vet and have her euthanized. Is that my only option. What is happening to her? What about my other girls? They are all looking and acting normal. Not sluggish at all. Any advice is welcomed!