Avian infectious Coryza is a common bacterial respiratory disease affecting chickens. It typically doesn’t create high mortality, but left unchecked is hard on a flock and easily transferred to other birds. In its worst form, Coryza is found with concurrent infections from mycoplasma and E. coli and can be fatal.
Coryza is spread through direct contact with infected birds and through contaminated surfaces and equipment.”
Source: Chicken Whisperer. Link. With no vaccine available to backyard flock owners focusing on prevention is the best use of time and energy. Infected birds that recover are likely still carriers of the bacteria that can infect other birds.