If its viral, then I would imagine that your whole flock has the problem, just that either possibly their immune systems can cope better than others. I guess its a bit like humans getting the flu or a cold - some suffer more than others. Each flock member may show symptoms at one time or another. I have this issue in my flock and they usually get over it after a week or so. I must stress however that i am by no means suggesting that problem IS viral.
All the best
I will just be so sad if I can't add any new chickens to my flock, we had planned on expanding next year after the chicks we got this fall are older. Possibly even hatching our own. I really wish I knew a good vet around here that I could have do some testing to find out what this is and how serious it is, but I feel like it will cost me an arm and a leg! I need to make some phone calls to the feed stores and see if they know of anyone. But money is a bit of an issue right now, we aren't broke but I also don't have any extra laying around to drop a bunch at the vet. I sure wish it grew on trees, or that maybe it could be laid by chickens! Well, I guess it sort of is when you sell your eggs haha. We have just now started getting enough to sell, so they aren't exactly paying for themselves yet!!