Flu viruses have a segmented genome which can get jumbled up during replication (gene mixing). This allows different strains to easily form from existing types. Flu occurs in seasonal epidemics and periodically as major pandemics.
Illustration ID: VIRUS-influenza-re-assort_op
copyright Russell Kightley
Illustration size: 24.4 Mpixels (69.7 MB uncompressed) - 4744x5135 pixels (15.8x17.1 in / 40.2x43.5 cm at 300 ppi)