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This image may not be used in educational posters .Illustration of influenza (flu) virus particles. Flu viruses are covered by spikes of haemagglutinin (H, slender knob-like projections) and neuraminidase (N, square-like projections). Flu viruses have a segemented genome which can get jumbled up during replication (gene mixing). This allows different strains to form easily from existing types. (KEYSTONE/SCIENCE PHOTO LIBRARY/RUSSELL KIGHTLEY)