Influenza virus particles. Flu viruses are covered by spikes of Haemagglutinin (slender knob-like projections) and Neuraminidase (the 4-part squarish-looking projections), hence the H and N names, such as H5N1 or H1N1. Flu viruses have a segmented genome which can get jumbled up during replication (causing gene mixing or reassortment). This allows different strains to easily form from existing types. Flu occurs in seasonal epidemics and periodically as major pandemics, with novel strains arising from reassortments. Often preventable by vaccination.
Two strains of influenza virus (blue and green) infect the same cell in a pig. Their segmented genomes reassemble (reassort) creating a new (red) pandemic strain. This daughter strain contains genome segments from both parental strains.