I too have a closed flock because of Coryza. I have given all but a few, multiple Tylan injections and for now, none of them have systoms. It is however quite a battle reintroducing treated chickens into the flock. I'm sure some have been weaked because of the disease and that causes the others to pick on them. Right now I have a Plymouth Rock in a separated part of the coop because the rooster is after her. I have decided that I will no longer treat for Coryza. I will just allow my flock to naturally die off as Coryza overtakes them. The thought makes me want to cry, but culling them is out of the question. I'm a vegetarian because I don't believe in killing animals. I do however want to build a Corya free coop. How far from the Coryza coop is a save distance to build? I have about 1/2 acre to build on. Thanks for any help you may be. Janice