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English: pone-0009025-g001: Human Influenza A reassortment events of the 20th Century.The reassortment events associated with human pandemics in the 20th century (adapted from Webster et al. [39]). A full complement of eight gene segments of zoonotic origin caused the 1918 Spanish flu. In 1957, the H2N2 Asian flu pandemic replaced the HA, NA and PB1 segments, and in 1968 the H3N2 Hong Kong pandemic replaced the HA and PB1 segments only [40]. In both cases, the new subtype fully replaced the subtypes previously circulating amongst humans. The 1977 Russian epidemic introduced an H1N1 strain almost identical to that circulating prior to 1957, and may have been caused by the release of 20-year old frozen viruses [19]. The H1N1 and HxN2 lineages have since co-circulated in the human population; recently, their reassortment has given rise to human strains of H1N2 subtype.