Read my byc page. CRD is not uncommon although some would like to make you think so. Don't cull your birds, the wild ones have it too. Not only that, no breeders or egg swap folks are MS clear- or atleast show paper work that they have been tested.
You should only give these guys Terramycin if she is showing signs of a secondary infection. Cough, sniffle, let it run its course until they are down, listless and not eating as much. Then don't wait to long, if it makes sense.
Good luck, and if you have one with it, they all have it but like usual, it will not show unless there are conditions like stress or weather changes can trigger this, like the common cold, its a virus and antibiotics do not help virus'.
I had a bird necropsied this summer to see what I missed in this birds death, and it was terramycin resistant pneaumonia. Please be careful.