From Renaissance Italy to Sydney: a short history of the influenza blues

Influenza gets its name from the Italian Renaissance when an outbreak of flu was thought to be due to the influence of a particular planetary alignment.

We now know there are three types of flu virus: influenza A, B and C, and within each type lies the potential for millions of subtypes.

The flu bug identified in these islands this winter is the H3N2 sub-type of influenza A. It is also known as Sydney flu because it was first identified there.

All viruses are able to mutate. They alter their genetic make-up in response to changing environmental circumstances. Influenza virus is especially good at this, which explains why you can catch flu more than once and also why it is necessary to vaccinate people annually.

Flu virus invades the cells of the human body by injecting a spike of its own cell wall into the cells of the respiratory tract. It then begins to multiply, using the human cell as a new home.

The virus reaches the respiratory tract via droplets in the air, which come from the coughs and sneezes of other flu victims. So crowded places such as buses and trains are ideal locations for the virus to spread. Hence the medical advice to go to bed for a number of days. This not only helps the flu victim but limits the spread of the virus.

Influenza reproduces rapidly in the throat and the lining of the lungs. From these initial sites in the respiratory tract it begins to invade the rest of the body by way of the bloodstream.

This phase is referred to by doctors as a viraemia and is the point at which the flu victim feels as if he or she has been hit by a sledgehammer. A high temperature, profound muscle aches and other pains follow. The body is now in the full grip of the flu virus.

The immune system begins to hit back almost immediately. In young, fit adults the flu is brought under control within a matter of days.

For those with a compromised immune system, however, recovery will be much slower. The elderly and those with heart and lung diseases are also at risk of the complications of influenza.

The principal complication is the development of a secondary bacterial infection, usually pneumonia. For somebody with pre-existing breathing problems, this is a serious illness which often requires hospitalisation. Pneumonia has traditionally been referred to as the old man's friend for the high mortality it causes in the elderly.

A rarer complication occurs when the virus invades the lining of the heart.

This is called pericarditis, a serious condition requiring specialist treatment.

A flu pandemic refers to a widespread concentration of infection spanning several regions. The last pandemic was in 1968 when the Hong Kong flu gripped Europe, the US and Asia. A pandemic causes more deaths among young, healthy people, reflecting the highly virulent nature of the viral strain.

The good news is that this year's flu falls well short of pandemic status.

Dr Muiris Houston has been appointed recently as Irish Times Medical Correspondent. He can be contacted by email at: mhouston@irish-times.ie