I vote Marek's, too. I deal with it in my flock. Some chickens have symptoms and some don't, but it is VERY common. Look up Range paralysis or Marek's. Some birds will recover if fed and watered. I lose a hen every now and them. Some are young, some live to be older and develop tumors. They are happy and spoiled in a backyard flock, so I don't stress about it.
I did have lab testing done after one passed away to confirm the diagnosis, and now I know what to do for the birds when they get sick without guessing.
I did have lab testing done after one passed away to confirm the diagnosis, and now I know what to do for the birds when they get sick without guessing.