Flu: a plague in history

Flu pandemics killed millions earlier this century

Flu is not thought a threat to young healthy people - but those who remember the pandemic of 1918 know otherwise.

Last year, approximately 20,000 people in the UK died from what were thought to be flu-related illnesses.

Yet even this was not an epidemic - far short of it in fact.

The Spanish flu pandemic which swept the world at the close of the First World War is believed to have accounted for the deaths of at least 40m, including 280,000 in the UK.

The virus made its way around the globe, helped by a conflict which led to large movements of people from one continent to another.

At first, the casualties of the illness, many of them young, healthy, men were dwarfed by those of the war itself, but the death toll continued to rise.

In all 600,000 people in the US died of flu-related illness.

The plague killed hundreds of thousands in the UK

It was dubbed Spanish flu because of the extraordinary death rates there - a total of 8m killed.

However, some of the lessons of that illness have been learned, and assisted the inception of worldwide monitoring of flu outbreaks so that early warning of the "next big one" would be available.

The most virulent strains of flu, according to scientists, appear to be those with a mixture of animal DNAs inside them.

This appears to be confusing for the human immune system, which takes longer to adapt, allowing the virus more opportunity to wreak havoc, weakening the body and clearing the path for other, more dangerous infections to take hold.

Since 1918, there have been two other recorded pandemics, in 1957 and 1968, both of which also killed millions around the world.

"People have no way of predicting when another novel virus will arrive," says Alan Hay, director of the WHO's influenza centre at the National Institute for Medical Research in London.

"All we can say is that it will happen."

While most flu viruses are subtle variations on existing viruses, those with the potential for pandemic are those which have made a more pronounced genetic shift.

People have no way of predicting when another novel virus will arrive

Alan Hay, WHO

This, coupled with the ability to infect humans easily, makes them very dangerous indeed.

There have been recent scares - an outbreak of flu from chickens in Hong Kong three years ago affected 18 people, and killed six, and sparked a massive slaughter of the birds.

However, the evidence suggested that while the flu was quite dangerous if contracted, it was not particularly communicable between humans.

The explosion of intercontinental travel means that new viruses could in theory be transmitted around the world in a matter of days.

However, advances in vaccination mean that the death toll would be much smaller.

The NHS' biggest concern is how it will be hit by the flu, itself an inevitable part of winter - but an entirely unpredictable one.

The flu season officially starts in October and flu levels will always peak at some time between then and March.

The problem, according to the Public Health Laboratory Service, which monitors flu levels, is that there is no way of telling how high or when that peak will be.

The last official flu epidemic was 11 years ago, but, unlike other diseases, flu is not cyclical so there is no way of guessing when the next epidemic might be.

Yes, it could be this winter, though the fact that no new strains of flu have emerged during the recent Southern Hemisphere winter does bode well.

"If there is a completely new strain then we start to worry," said a PHLS spokesman.

"But the thing with flu is there is just no pattern. We don't know whether the rate will be 50 per 100,000 or 500."

It is that unknown that could spell crisis for the NHS this winter.