I've only had chickens for 4 months and I wish I had known it was so unsafe to introduce new started birds to a flock, I was trying to be a good samaritan and rescued an abused hen. I separated her but unfortunately I didn't keep her separate for long enough. because once I was "sure" that she was ok acting normal and healthy I put her in with my other completely healthy flock. 2 days later she came down with what looked like a head cold and puffy eyes, turns out she has all the symptoms of coryza and now my whole flock is infected and will become carriers I am treating them one by one as the symptoms show up with injectible Tylan 50. I feel terrible that I unknowingly brought this disease to my undeserving hens and roos and now can not bring any new chicks onto my property until all infected birds are dead and the area cleansed.