Avian influenza or 'bird flu' is a highly contagious disease of birds, caused by influenza A viruses. In birds, the viruses can present with a range of symptoms from mild illness and low mortality to a highly contagious disease with a near 100% fatality rate.
The bird flu virus currently affecting poultry and some people in Asia and other areas is the highly pathogenic H5N1 strain of the virus.
As the virus can remain viable in contaminated droppings for long periods, it can be spread among birds, and from birds to other animals, through ingestion or inhalation.
All bird species are thought to be susceptible to avian influenza. Migratory birds such as wild ducks and geese can carry the viruses, often without any symptoms of illness, and show the greatest resistance to infection. Domestic poultry flocks, however, are particularly vulnerable to epidemics of a rapid, severe and fatal form of the disease.