Coryza had a foul smell to the breath of infected chickens. Like rotting flesh. You will be able to recognize it instantly once you smell it, and you will never, EVER forget it.
I brought home a chicken with coryza once. It resulted in three long days of suffering for me. I had quarantined, and I noticed my rooster smelled kinda funny. And his breathing sounded odd, like he was breathing through a straw with smoothie in it. I did some research and found he had coryza.
The next day, I got up very early in 20 degree weather and several feet of snow to get rid of him. After that, I spent an hour freezing and shivering and wet, cleaning up every last trace of coryza-infected manure.
I spent the next two days worried sick about my laying flock, for although I had quarantined the bird, I had not practiced proper sanitation in tending to my laying flock FIRST, and then the rooster.
They were all right.
Coryza is NASTY, and it will never go away, even if the bird recovers. Stress will bring out another bout, with more fatalities. So much suffering for the chickens.