The 1957 Asian Flu
The "Asian Flu" was a category 2 flu pandemic outbreak of influenzavirus A that originated in China in early 1956 lasting until 1958. Some authors believe it originated from mutation in wild ducks combining with a pre-existing human strain. Other authors are less certain. The virus was first identified in Guizhou. It spread to Singapore in February 1957, reached Hong Kong by April, and US by June. Death toll in the US was approximately 69,800. Estimates of worldwide deaths caused by this pandemic varies widely depending on source; ranging from 1 million to 4 million, with WHO settling on "about 2 million".
Asian Flu was of the H2N2 subtype (a notation that refers to the configuration of the hemagglutinin and neuraminidase proteins in the virus) of type A influenza, and an influenza vaccine was developed in 1957 to contain its outbreak. The Asian Flu strain later evolved via antigenic shift into H3N2 which caused a milder pandemic from 1968 to 1969.
Both the H2N2 and H3N2 pandemic strains contained avian influenza virus RNA segments. While the pandemic human influenza viruses of 1957 (H2N2) and 1968 (H3N2) clearly arose through reassortment between human and avian viruses, the influenza virus causing the 'Spanish flu' in 1918 appears to be entirely derived from an avian source.