How avian influenza virus is spread
Water birds such as wild ducks are believed to be the carriers of all avian influenza type A viruses. The viruses are carried inside the birds' intestines and are distributed into the environment via bird faeces (poo). Migratory birds infected with the virus could potentially spread the bird flu to any of the countries they visit.
Wild birds don't usually show symptoms of bird flu, although the currently circulating H5N1 strain has caused illness and death in some wild birds. The avian influenza virus can more frequently kill domesticated birds, such as chickens and turkeys.
Symptoms in birds differ according to the species but can include diarrhoea, breathing difficulties, swollen head and death. A sick bird sheds the virus in its feathers, mucous, saliva and faeces.
Humans who have close contact with sick birds are at risk of infection with bird flu. For example a person may handle a sick bird, contaminate their hands with chicken faeces, and forget to wash their hands before eating. They will then ingest the infected bird faeces. This is the most common way for a human to catch the bird flu. The virus can also survive in raw poultry meat but is destroyed during normal cooking.
There is no evidence that the current circulating H5N1 strain of bird flu can be spread easily from human to human.