Hi, here's a response i wrote a month or so back to another member experiencing similar problems. It's hardly a scholarly article but I hope that it may help a little...
There are a number of schools of thought on what to do - I'm sure there may be others BTW
1. Take your chicken to the vet and go from there
2. Dose the whole flock with broad spectrum antibiotics or if you can get one that is for respiratory infections then all the better (they will not kill a virus, but can help stop secondary infections)
3. Give the flock vitamins / electrolytes in their water to help give their immune system a boost and see how they go.
Theres no reason not to begin on the vitamins ASAP.
Some respiratory infections are only as harmful as humans getting a cold, others a lot more serious. Assuming your flock survive and get over the illness, bear in mind that they may be carriers of the illness for life and will pass it to new flock members. It would be unwise to sell or give any of your flock to other chicken keepers in the future.
Its almost impossible to diagnose exactly what respiratory problem it is without a vet's expertise, but ultimately, the methods / options of dealing with them remain the same (sans vet)
All the best