Newcastle disease is a viral disease of birds caused by avian paramyxovirus 1 (APMV-1). For official control purposes, this disease is currently defined as the most severe form of the illness, which is caused only by certain viral strains. Many less virulent strains of APMV-1 also circulate among domesticated and wild birds. These viruses usually cause much milder clinical signs or infect birds asymptomatically. However, they can sometimes evolve to become the highly virulent strains that cause Newcastle disease.

Newcastle disease is considered to be one of the most important poultry diseases in the world. Chickens are particularly susceptible, and may experience morbidity and mortality rates up to 100%. Outbreaks can have a tremendous impact on backyard chickens in developing countries, where these birds are a significant source of protein and this disease is endemic. In developed countries, where highly virulent APMV-1 strains have usually been eradicated from poultry, trade embargoes and restrictions cause significant economic losses during outbreaks. Newcastle disease can also affect other commercial poultry, game birds, ratites, and various pet, hobby and zoo birds. Some of these birds become ill, while others carry and shed virulent viruses asymptomatically. Subclinically infected birds, particularly illegally imported psittacines, can introduce Newcastle disease into countries where it does not usually exist.

A number of recent studies have examined the epidemiology of APMV-1 in wild birds. Although these birds are mainly infected with low pathogenicity strains of APMV-1, highly virulent strains circulate in some cormorant populations in North America. Outbreaks occur periodically in cormorants, with severe illness and deaths in young birds. Viruses from cormorants can also infect nearby gulls, and could spread to other wild or domesticated birds. Strains of APMV-1 maintained in wild (and domesticated) Columbiformes may also be a concern, although these particular viruses generally tend to cause serious disease only in pigeons and doves. Recently, several papers described sporadic infections with virulent APMV-1 viruses in various wild birds throughout the world. The significance of this finding is still uncertain.