Image of an influenza virus particle, the cause of flu. These viruses are covered by spikes of Haemagglutinin (red projections) and Neuraminidase (the squarish yellow projections), hence the H and N names, such as H5N1. These spikes pass through the greenish viral envelope to dock with the underlying matrix (M) proteins shown in grey. 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.