Reassortment is responsible for some of the major genetic shifts in the history of the influenza virus. The 1957  and 1968 pandemic flu strains were caused by reassortment between an avian virus and a human virus , whereas the H1N1 virus responsible for the 2009 swine flu pandemic has an unusual mix of swine, avian and human influenza genetic sequences.
Reassortment is responsible for some of the major genetic shifts in the history of the influenza virus. In the 1957 "Asian flu" and 1968 "Hong Kong flu" pandemics, flu strains were caused by reassortment between an avian virus and a human virus . In addition, the H1N1 virus responsible for the 2009 swine flu pandemic has an unusual mix of swine, avian and human influenza genetic sequences.
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