by Darian Peters

Created on: February 06, 2009   Last Updated: May 30, 2009

Influenza is a viral infection that is particularly common and, in the worst cases, potentially deadly. Most of us will have experienced the disease at some point in our lives. As a seasonal illness this will often have been in the winter. The illness will typically just have put us out of action for a few days with symptoms ranging from chills, coughs, fever, and headaches to muscle pain and pharyngitis. Indeed significant numbers of people experience it on multiple occasions. But how can a disease that many people think of as nothing more than an extreme version of the common cold end up killing someone?

There are five types of virus that cause influenza and these are Influenzavirus A, Influenzavirus B, Influenzavirus C, Isavirus, and Thogotovirus. They are RNA viruses from the family of viruses called Orthomixoviridaes. Different types and strains effect a different range of organisms, including humans, other mammals, and also birds. The range of organisms effected can change over time as the virus evolves. Many people are worried that the H5N1 bird flu will mutate into a form that leads to a pandemic in humans, for example.

Different strains of the virus have different physical structures and this can give them different characteristics in terms of symptoms and infection rates. An influenza epidemic will typically kill around 0.1% of those infected but in some particularly nasty cases this can be dramatically higher. The Spanish Flu pandemic of 1918, for example (which was caused by an Influenzavirus A type virus called H1N1 with an infection rate of 50%) had an estimated mortality rate of between 2 and 20%. This is estimated to have led to the deaths of anything from 40 million to 100 million people worldwide. This was an unusual epidemic in that 99% of those who died were young adults rather the very young or old.

Deaths can be caused in several ways, either directly by the virus or through secondary infection. The influenza virus can cause direct death through the tissue damage that it is responsible for. This can involve haemorrhages (bleeding through broken blood vessels) and oedema (an accumulation of fluid) in the lungs, for example.

But influenza can also cause death through secondary infection by bacterial pneumonia, for example. This is especially true in the cases of particularly vulnerable people such as young children and the elderly and those with weakened immune systems, such as HIV patients.

But on another level of analysis various failures can contribute to people dying from flu. The influenza virus can be killed by detergents and disinfectants so poor hygiene, not using these products, and failing to wash your hands can increase risk of infection in the first place. Having a generally fit and healthy lifestyle can maintain your immune system in a stronger position to fight the disease (although in the case of Spanish Flu it could be argued that those with stronger immune systems, young adults, were more at risk because the damage was being caused by an immune system over-reaction). Also, if you contract influenza, it is important to look after yourself and see a doctor if the symptoms seem to require it. Certainly several days in bed resting whilst getting plenty of liquids and no alcohol or tobacco are advised.

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