Sorry to hear about your chicken having problems. We lost a chicken this morning about 3.5 years old, and she had Marek's disease. We have a very good vet in Petaluma who diagnosed her. We got her from Tractor Supply who didn't do a good job of vaccinating our chickens for Marek's disease, so about half our flock has/had Marek's disease. We got the other half of our flock from someplace elae and they are doing fine. Marek's disease can manifest itself by affecting the way a chicken walks, or gets in the abdomen/ovaries and cause cancer and/or water belly. But the chicken that died has limped from Marek's disease for at least a year. It would be odd that she suddenly got Marek's disease and seems to be dying. I think the only way to really know, is to take her to a vet and get a diagnosis. I know it's expensive and time consuming, but that is probably the only way to be sure.