I think what we're looking at is MG...mycoplasma gallisepticum, there are many different strains of avian mycoplasma and vary in virulence. You can also be dealing with secondary infections as well.
Morbidity is high but mortality is low. Once they become infectious and if they recover, they will be carriers for life.
It's not Infectious Coryza, that is an acute respiratory infection of chickens.
Treatment consists of several medications/antibiotics, including chlortetracycline, oxytetracycline, erythromycin, spectinomycin, tiamulin, tylosin and enrofloxacin. The antibiotics may lessen the symptoms but do not eliminate the infection. Most of these antibiotics are by a script from a veterinarian.
The best prevention is a strict biosecurity and sanitation regime.