If they are separated only by wire, then if what they have is contagious it would easily spread from one pen to another. Chickens often will pick off and eat the feathers of others in the flock. It's usually a sign that they need more to pick on or scratch through. Once they taste blood they will pick at the rear ends of each other and they will bleed to death quickly. I'd recommend making sure they have lots of things to pick and scratch on. Like sod, Alfalfa hay, or you can buy a block of compressed mixed seeds that they can pick on. A combination of a couple of those options would be good. Chickens that are locked in all the time are more prone to doing this. If the dead chicken has a red and swollen vent in the back, it's a good sign that is what's happening. A picture of the back end of a dead one would be helpful, I know it would be gross to look at, but I could easily tell by seeing it.
