Avian influenza, or bird flu for short, is an infectious disease affecting birds including domestic poultry and wild fowl. It is caused by a Type A influenza virus. Birds affected by the virus will exhibit either mild or severe symptoms. In mild cases the respiratory tract and alimentary canal are affected and the mortality rate is low. In severe cases symptoms are more general and include hemorrhaging and septicemia with high mortality rates. This difference in symptoms is largely determined by the virulence of different strains of the avian influenza virus.

Bird flu, can be classified into three types based on variations in the toxicity of its viruses and their pathogenic affects. One type is classified as highly pathogenic. There is also a type with low pathogenic effects and one which is non-pathogenic. There are different subtypes. Where the disease is caused by subtypes of the H5 and H7 viral strains, for example H5N1 and H7N7, it is termed highly pathogenic avian influenza (hereinafter referred to as HPAI). The current epidemic both at home and abroad is caused by the H5N1 subtype.  Given its high morbidity and mortality the effects could be potentially devastating.

(China.org.cn February 13, 2004)