Cutaway model of an influenza virus particle showing internal structures. Superficially you can see the spikes of Haemagglutinin and Neuraminidase. These spikes pass through the viral envelope to dock with the underlying matrix (M) proteins. Inside the matrix shell you can glimpse the ribonucleoproteins that house the viral genome. Flu viruses have eight of these RNPs. If two or more strains infect the same cell, then progeny viruses can incorporate segments from more than one parent leading to new and possibly more dangerous strains. This is called genetic reassortment. Flu occurs in seasonal epidemics and periodically as major pandemics.