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.
The widespread persistence of the H5N1 [a highly pathogenic disease-causing
strain of avian influenza] in poultry populations poses two main risks for
The first is the risk of direct infection when the virus passes from poultry
to humans, resulting in very severe disease. Of the few avian influenza viruses
that have crossed the species barrier to infect humans, H5N1 has caused the
largest number of cases of severe disease and death in humans.
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. (…)
A second risk, of even greater concern, is that the virus - if given enough
opportunities - will change 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). (...)
Hong Kong has experienced two outbreaks in the past. In 1997, in the first
recorded instance of human infection with H5N1, the virus infected 18 people and
killed 6 of them. In early 2003, the virus caused two infections, with one
death, in a Hong Kong family with a recent travel history to southern China.