Influenza virus particle, the cause of flu. These viruses are covered by spikes of Haemagglutinin (slender knob-like projections) and Neuraminidase (the squarish looking ones), hence the H and N names, such as H5N1. Flu viruses have segmented genomes which can get jumbled up during replication (gene mixing or reassortment). This allows different strains to easily form from existing types. Flu occurs in seasonal epidemics, and periodically as major pandemics.