Avian influenza, or "bird flu", is a contagious disease of animals caused by viruses that normally infect only birds and, less commonly, pigs. Avian influenza viruses are highly species-specific, but on rare occasions have crossed the species barrier to infect human beings.
In domestic poultry, infection with avian influenza viruses causes two main forms of disease, distinguished by low and high extremes of virulence. The so-called low pathogenic form commonly causes only mild symptoms (ruffled feathers, a drop in egg production) and may easily go undetected. The highly pathogenic form is far more dramatic. It spreads very rapidly through poultry flocks, causes disease affecting multiple internal organs, and has a mortality that can approach 100%, often within 48 hours.
Currently, the highly pathogenic avian influenza A(H5N1) is endemic among poultry in some parts of the world, and has very high mortality in poultry flocks. On multiple occasions since the virus started spreading across Asia in 2003, it has crossed the species barrier and caused severe disease and death in human beings.
The continuing cases of bird-to-human transmissions provide opportunities for the virus to adapt, either through reassortment or adaptive mutation, so that sustainable transmission among humans occurs. This increases the risk that avian influenza A(H5N1) may cause a human pandemic. While it is far from certain that the A(H5N1) virus will ever adapt in this way, the presence and the spread of avian influenza have raised awareness of the risk of a human influenza pandemic.