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Influenza is caused by an RNA virus of the orthomyxoviridae family. There are three types of these viruses and they cause three different types of influenza: type A, B and C.

  • Influenza A viruses infect mammals (humans, pigs, ferrets, horses) and birds. This is very important to mankind, as this is the type of virus that has caused worldwide pandemics.

  • Influenza B viruses infect only humans. It occasionally causes local outbreaks of flu and is usually confined to youngsters.

  • Influenza C viruses also infect only humans. They infect most people when they are young and rarely causes serious illness
The reason why the A type of influenza virus is the type most likely to cause epidemics and pandemics is because the influenza A virus can undergo antigenic shift (is the process by which two different strains of influenza combine to form a new subtype) and present a new immune target to susceptible people.

Populations tend to have more resistance to influenza B and C because they only undergo antigenic drift (the natural mutation over time of known strains of influenza to evade the immune system) and have more similarity with previous strains.

Where a finer grained classification of the virus strain is needed, this is done by reference to the structural forms of two viral proteins (haemagglutinin and neuraminidase) that are essential to the virus' life cycle. Thus one might speak of H1N1 or H3N2 viral strains.

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