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Influenza: A case study
Influenza: A case study

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1 Background to the case study

Influenza is a myxovirus belonging to the family of viruses known as Orthomyxoviridae. The virus was originally confined to aquatic birds, but it made the transition to humans 6000–9000 years ago, coinciding with the rise of farming, animal husbandry and urbanisation.

These changes in human behavior and population density provided the ecological niche that enabled influenza, as well as a number of other infectious agents such as the viruses that cause measles and smallpox, to move from animals and adapt to a human host.

Influenza as a disease has been recognised for centuries, even though the viruses which cause it were not correctly identified until the early 1930s, first in the UK and then in the USA. Indeed the name itself is derived from an Italian word meaning ‘influence’, and reflected the widespread belief in medieval times that the disease was caused by an evil climatic influence due to an unfortunate alignment of the stars.

Our current understanding – that infectious diseases are caused by infectious agents – is so ingrained that such mystical causes for an illness now seem absurd. However, even during the Middle Ages, people had a sound idea of infection and realised that some diseases could be passed from one individual to another and others could not. For example, the use of quarantine for a disease such as plague, but not for many other illnesses, shows that people could distinguish infectious diseases from non-infectious diseases even if the causative agent and the method of transmission were obscure.

The idea that influenza is caused by the influence of the stars, though not a satisfactory explanation of how the disease spread, does identify an important feature of flu – that serious epidemics of the disease occur at irregular intervals.

For example, in the twentieth century there were at least five major epidemics of flu that spread around the world (a pandemic), and there were less serious epidemics in most years. In times when people believed in the spontaneous generation of life, the stars would have seemed a reasonable explanation for unpleasant and unexpected epidemics.