Infectious Coryza is an acute, highly contagious respiratory disease caused by the haemophilus paragallinarum bacteria. The disease is found in chickens and sometimes other fowl, such as pheasants and guinea fowl. (Claims that quail can be infected are inaccurate, as that is a similar but different disease).
In humans, coryza is basically the common cold. In chickens, coryza is pretty much a death sentence. Infectious coryza has a high mortality rate and survivors of the illness remain carriers for life. They are risks to the rest of the flock and any new chickens introduced to them, plus management of coryza requires regular medication to be given the chickens regularly, so most experienced chicken keepers cull the survivors anyways (especially in big laying operations where the risk of infecting a flock means the risk of infecting a hundred other chickens).
There are treatments for coryza if culling is too unsavory an option; since it is a bacterial disease, certain antibiotics can help get rid of symptoms (don't trust over the counter stuff though. Nine times out of ten, it's useless), but again, the surviving hen will still be a carrier for life and it runs the risk of relapse at any time. The internet is full of horror stories of chicken keepers having to cull their entire flock because they accidentally introduced a seemingly healthy chicken, only for it to turn out that the new chicken was actually a coryza carrier the seller sold anyways.
There are also preventative vaccines you can get from vets to avoid the trouble all together.
Signs of infectious coryza include:
* Swollen eyes/head/wattles.
* Putrid-smelling nasal and eye discharge.
* Shortness of breath/heavy breathing.
* Lack of appetite.