Figure 5
Evolution and reassortment of human Influenza A viruses. This figure (adapted from [3]) shows how human-transmissible Influenza A subtypes were acquired from the avian pool during 20th Century pandemics. A full complement of eight gene segments of avian origins originated the 1918 Spanish flu, while the following two pandemics followed the acquisition of a smaller number of avian genes through recombination. In 1957, the H2N2 Asian flu replaced the HA, NA and PB1 segments, while the H3N2 Hong Kong pandemic of 1968 replaced the HA and PB1 segments only. In each of these pandemics, the new subtype fully replaced the previously circulating subtype. The minor Russian pandemic of 1977 was caused by the reintroduction of a H1N1 strain almost identical to that circulating prior to 1957, leading to the widely-held view that it was caused by the release of 20-year old frozen viruses. The H1N1 strain has not supplanted H3N2, and the two lineages co-circulate in the human population to the present day; the recently emerged H1N2 subtype has arisen from their reassortment. All currently circulating PB2 proteins are therefore thought to have descended from the Spanish flu strain, although the PB2 protein associated with HxN2 has diverged significantly from that of the H1N1 lineage.