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 have, on rare occasions, crossed the species barrier to infect humans.
A highly pathogenic avian influenza virus, as the current H5N1 virus circulating in South-East Asia, poses two main risks for human health.
The first is the risk of direct infection when the virus passes from poultry to humans, resulting in very severe disease. Unlike normal seasonal influenza, where infection causes only mild respiratory symptoms in most people, the disease caused by H5N1 follows an unusually aggressive clinical course, with rapid deterioration and high fatality. Primary viral pneumonia and multi-organ failure are common.
A second risk, of even greater concern, is that the virus â€“ if given enough opportunities â€“ will reassort into a form that is highly infectious for humans and spreads easily from person to person. Such a change could mark the start of a global outbreak (a pandemic).
Reference: WHO Health Topics : Influenza and Avian Influenza