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Impact and challenges of the flu virus

There are three types of influenza viruses – A, B and C – but only types A and B cause seasonal epidemics in humans. Type A viruses can also cause pandemics and are classified into subtypes according to the variety and combinations of two surface proteins, hemagglutinin (HA) and neuraminidase (NA). Circulating flu viruses continuously evolve, so seasonal vaccines contain a mix of two influenza A subtypes and one or two influenza B viruses, which are reviewed each year by the World Health Organization (WHO).