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• Influenza:
• is an acute viral infection that spreads easily from person to person;
• circulates worldwide and can affect anybody in any age group;
• causes annual epidemics that peak during winter in temperate regions;
• is a serious public health problem that causes severe illness and death in higher-risk populations.
• An epidemic can take an economic toll through lost workforce productivity, and strain health services.
• Vaccination is the most effective way to prevent infection.

There are three types of influenza viruses (A, B and C), based on antigenic differences of their nucleo- and matrix proteins. Avian influenza viruses belong to type A. Influenza A and B viruses cause epidemics among human beings, while influenza C causes only mild disease. For this reason, surveillance is usually performed for influenza A and B viruses only.

Influenza A viruses can be subdivided into subtypes based on the antigenicity of two different protein components, called antigens, on the surface of the virus. They are spike-like features called haemagglutinin (H) and neuraminidase (N) components. They can elicit subtype-specific immune responses, which are fully protective within but only partially protective across different subtypes. Influenza A viruses currently cluster into sixteen H (H1–H16) and nine N (N1–N9) subtypes.

The H subtypes are epidemiologically most important, as they govern the ability of the virus to bind to specific receptor type(s) of a certain host and enter cells, where multiplication of the virus then occurs. The N subtypes govern the release of newly formed virus from the cells.