History and evolution of HPAI viruses in southeast Asia.

Highly pathogenic avian influenza (HPAI) has been recognized as a serious viral disease of poultry since 1878. The number of outbreaks of this disease globally has increased in the past 10 years culminating in 2004 with the unprecedented outbreak of H5N1 HPAI involving nine countries in East and South East Asia. Apart from the geographical extent of this outbreak and apparent rapid spread, this epidemic has a number of unique features, among which is the carriage of highly pathogenic AI viruses by asymptomatic domestic waterfowl.