Backyard chickens explains what avian pox is, how the birds get it and how to prevent it. I had not heard of this and it seems there are a couple different strains of it.
http://www.backyardchickens.com/a/avian-pox-how-to-treat-your-chickens-for-avian-pox
The Chicken Health Handbook
Treating the Sick or Injured Chicken