Avian influenza (禽流感), 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 many subtypes of avian influenza viruses, but only some strains of four subtypes have been highly pathogenic in humans. These are types H5N1, H7N3, H7N7, H7N9, and H9N2.
On March 30, 2013, three cases of human infection with influenza A (H7N9) were confirmed in Shanghai and Anhui Province.
On Dec. 18, 2013, a 73-year-old woman in Jiangxi Province died of pneumonia from the H10N8 strain, the first human fatality confirmed to be caused by that strain.
By the end of 2013, a total of 146 human cases of H7N9 have been confirmed in the Mainland, including Zhejiang (52 cases), Shanghai (34 cases), Jiangsu (28 cases), Guangdong (six cases), Jiangxi (six cases), Fujian (five cases), Anhui (four cases), Henan (four cases), Beijing (two cases), Hunan (two cases), Shandong (two cases) and Hebei (one case).