Influenza A virus

　　Influenza A virus [File photo]

Influenza A virus causes influenza in birds and some mammals. There are two main types of this strain of influenza: avian flu and swine flu, including the highly pathogenic H5N1, H7N9 and H1N1 strains. Influenza A/H1N1 is the subtype of influenza A virus that was the most common cause of severe human influenza in 2009. By December 31 of 2009, over 120,000 people had been infected with the virus and 648 of them died, according to the Chinese Ministry of Health. The H5N1 strain has been spreading in Asia for years since it was found to have infected a human in Hong Kong in 1997.