Animation of an influenza virus particle using a cutaway model showing internal structures. Haemagglutinin (red) and Neuraminidase (yellow) spikes pass through the green viral envelope to dock with the underlying matrix (M) proteins shown (purple). Inside the matrix shell are the RNPs (ribonucleoproteins) containing viral genes. Flu viruses have eight separate RNPs. If two or more virus strains infect the same cell, progeny viruses can incorporate segments from more than one parent leading to new and possibly more dangerous strains. This is jumbling up of genes is called genetic reassortment and is why flu is so variable and why new strains crop up regularly. It can also mix up flu strains from different species such as pigs, birds, and humans. Flu occurs in seasonal epidemics and periodically as major pandemics.

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