Infectious Coryza is caused by the bacterium Haemophilus paragallinarum and is seen in many countries especially in multi-age farms that are never depopulated. Morbidity is high but mortality low if uncomplicated, although it may be up to 20%.

The route of infection is conjunctival or nasal with an incubation period of 1-3 days followed by rapid onset of the disease over a 2-3 day period with the whole flock affected within 10 days. This results in increased culling. Carriers are important with transmission via exudates and by direct contact.