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-life-cycle_op
copyright Russell Kightley
Illustration size: 8.6 Mpixels (24.6 MB uncompressed) - 2744x3135 pixels (9.1x10.5 in / 23.2x26.5 cm at 300 ppi)