Two influenza viruses of different subtypes or genetic lineages, such as the human and avian lineages depicted in the video, may infect the same cell in various susceptible mammalian or avian species. During replication in the cell, gene segments from each virus may be mixed together and randomly assembled into the progeny virus.

The virus that emerges from the cell contains a new combination of gene segments with the surface proteins (HA and NA) new to the human or other susceptible population.  If the novel virus can successfully infect and spread among individuals within susceptible populations, outbreaks, epidemics, or even pandemics may occur.