The natural reservoir is wild and domesticated birds.
Transmission is by inhalation of infectious aerosols.
Intensive conditions favor spread by contact, on inanimate objects and airborne between poultry houses. Human infections occur mainly amongst laboratory workers and those who work with infected chickens or who give live vaccine, especially by aerosol. Egg production ceases. Petechial hemorrhages are characteristic, especially in the preventricular mucosa. Congestion and mucoid exudate appears in the lungs and bronchi.