Influenza, commonly known as the flu , is an infectious respiratory disease caused by the Influenza virus. Influenza viruses belong to the orthomyxoviridae family. The influenza virus contains eight negative-sense single-stranded RNA segments and is characterised by two surface proteins, hemagglutinin and neuraminidase.
Influenza A viruses have been causing infections of human populations for more than a century; H1N1 Spanish Flu (1918), H2N2 Asian flu (1957), H3N2 Hong Kong flu (1968), and most recently the pandemic in 2009 caused by a mutation of H1N1. Together they have cost over a 100 million lives. And even know, the flu causes hundreds of thousands of deaths yearly.