The 1918 flu (a subtype of H1N1 from 191820), Asian flu (a subtype of H2N2 rom 195758), and Hong Kong flu (a subtype of H3N2 from 196869) are some of the major modern influenza (an infectious disease of birds and mammals) pandemics.
An influenza pandemic is an epidemic of an influenza virus that spreads across a large region (either multiple continents or worldwide) and infects a large proportion of the population. There have been five in the last 140 years, with the 1918 flu pandemic being the most severe; this pandemic is estimated to have been responsible for the deaths of 50100 million people. The Hong Kong flu was a category 2 flu pandemic caused by a strain of H3N2 descended from H2N2 by antigenic shift, in which genes from multiple subtypes reassorted to form a new virus. The Hong Kong flu, also known as the 1968 flu pandemic, was a flu pandemic whose outbreak in 1968 and 1969 killed an estimated 14 million people globally. It was caused by an H3N2 strain of the influenza A virus, which is descended from H2N2 through antigenic shift, a genetic process in which genes from multiple subtypes are reassorted to form a new virus. The Asian flu was a category 2 flu pandemic outbreak of avian influenza that originated in China in early 1956 lasting until 1958. The 19571958 Asian flu pandemic, was a global pandemic of influenza A virus subtype H2N2 that originated in Guizhou, China. The strain of virus that caused the pandemic, influenza A virus subtype H2N2, was a recombination of avian influenza (probably from geese) and human influenza viruses. Influenza, commonly known as "the flu", is an infectious disease caused by an influenza virus. Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells, typically in the nose, throat, and lungs of mammals, and intestines of birds (Stage 1 in infection figure). 
