Influenza A viruses are constantly changing. This change accounts for seasonal epidemics, infrequent pandemics, and zoonotic outbreaks. A major mechanism underlying the genetic diversification of influenza A virus is reassortment of intact gene segments between coinfecting viruses. This exchange is possible because of the segmented nature of the viral genome. Here, I first consider the constraints and drivers acting on influenza A virus reassortment, including the likelihood of coinfection at the host and cellular levels, mixing and assembly of heterologous gene segments within coinfected cells, and the fitness associated with reassortant genotypes. I then discuss the implications of reassortment for influenza A virus evolution, including its classically recognized role in the emergence of genetically “shifted” pandemic strains as well as its potential role as a catalyst of genetic drift.

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