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Influenza is an illness of the respiratory tract caused by one of a number of influenza viruses. There are three virus types: influenza A, B and C. Influenza C causes only mild and usually sporadic respiratory illness. Regional and widespread epidemics are most often attributed to influenza A or B. Influenza A – which is found in humans and animals – causes the most severe disease and is the only type known to cause influenza pandemics.
Influenza A and B viruses have two main proteins on the outside of the virus: haemagglutinin (HA), which helps the virus enter the host respiratory cells, and neuraminidase (NA), which facilitates the release of virus particles from infected host cells. These proteins (also called antigens) are used in naming various viruses; for example, H5N1 is avian influenza or ‘bird flu’. H1N1 is also known as ‘swine flu’ and was the cause of the 2009 pandemic.
Influenza viruses have a high mutation rate – where the H and N antigens undergo change. Small mutations (called antigenic drift) are common and every 1–2 years virus change is seen. These changes are the cause of seasonal flu epidemics. The changes to the virus mean that little immunity is gained from previous infection, exposure or vaccination. This is why a new seasonal influenza vaccine is required each year.
Large mutations (called antigenic shift) cause the emergence of a new virus and the potential for a pandemic as there is no immunity in the population. Without any immunity, the virus can spread quickly from person to person, worldwide.
Three distinct influenza scenarios may be encountered in general practice:
- seasonal influenza, which occurs each winter. Most people experience 1–2 weeks of symptoms that are unpleasant but not usually life-threatening, except in the very young, pregnant women, or people with chronic health diseases
- epidemic influenza, which occurs when a new highly pathogenic and more severe influenza strain emerges. This can result in increased mortality and morbidity in local populations, especially in at-risk groups
- pandemic influenza, which occurs when a new highly pathogenic influenza strain emerges and spreads globally.