The BBC's Fiona Gammie on flu pandemics

The worst one this century was in 1918 and experts fear another one could be just around the corner.

The 1918 Spanish flu pandemic killed an estimated 40 million people - more people than died in the First World War - and was the biggest infection in history.

Many of its victims were young, fit people. The virus killed in an unusual way, by flooding the lungs with fluid and causing haemorrhaging.

It died out as mysteriously as it began.

But despite the pandemic's impact, scientists still do not know what caused it.

One expert called it "an 80-year-old murder mystery".

Imminent

Scientists believe that finding out what caused the pandemic could help prevent future outbreaks.

The next pandemic is thought to be imminent.

Jeffery Taubenberger, a US military pathologist, said: "It is 100% sure that we will have a pandemic in the future."

The nearest the world came to a pandemic in recent years is thought to be the Hong Kong avian flu of 1998.

Six people died and hundreds fell ill after the virus was passed on from chickens to man.

The virus in the chickens caused mass haemorrhaging, leaving them in a bloody pulp.

The authorities reacted quickly, removing the chickens in plastic bags.

Fears that the world may not react so quickly to another potential pandemic, leading to millions of deaths, are behind several recent attempts to trace the source of the 1918 pandemic.

Unlocking the secrets

Two attempts are covered in a BBC Horizon programme.

The two could not be more different.

Dr Jeffery Taubenberger: there will definitely be another flu pandemic

One involves a �250,000 high-publicity search by a team of experts on a Norwegian island.

The other is a lone attempt by a pathologist who began work on the pandemic in the 1950s.

Both are trying to find tissue from the preserved bodies of people who died from the 1918 pandemic in order to unlock its secrets.

Dr Kirsty Duncan, a geographer and Highland dancer, was inspired to set up the Norwegian expedition after reading a book by Dr Johan Hultin - the man behind the Alaskan attempt.

Dr Hultin first went to Alaska in the 1950s.

He wanted to find lung tissue from dead victims of the 1918 pandemic and believed the permafrost in Alaska would have preserved the bodies so that tissue could be taken.

He took lung tissue back to his laboratory with the intention of trying to make a flu vaccine.

The tissue was injected into the nostrils of ferrets in the hope that they would catch the 1918 virus.

However, the experiment failed because the virus was not live.

Dr Duncan read about his attempts and was inspired to collect together a "dream team" of scientific experts on flu to try to recreate Dr Hultin's experiment.

She tracked down the death records of seven miners who were buried in the permafrost on a Norwegian island.

Although some of the experts were worried that bringing the virus into the world of the 1990s could be dangerous if it was not totally isolated, the experiment went ahead.

A team of experts tried to dig up the bodies and managed to obtain some tissue samples.

Young soldier

At the same time as Dr Duncan's team was at work, Jeffery Taubenberger was searching military archives to find out more about the 1918 pandemic.

Dr Kirsty Duncan: inspired by Dr Holten to crack the flu mystery

Eventually, he tracked down one soldier, Private Roscoe Vaughan, who died on his way to the Western Front.

He had contracted a fever of 103 degrees, pneumonia and chills and turned blue.

Within five days of becoming ill, he was dead.

Dr Taubenberger found that his left lung had traces of pneumonia, but his right lung still showed the initial stages of infection.

This was important for Dr Taubenberger's attempts to trace the genetic sequence of the virus.

His team tried to rebuild the genes in the virus' make-up, but some were missing.

Dr Taubenberger needed more tissue. He published an article about his research and was surprised to receive a response from Dr Hultin.

Dr Hultin volunteered to return to Alaska in 1997 to try to find more tissue.

He got permission from native Americans living there to dig up some of the bodies of people who died in the 1918 pandemic.

Dr Holten's expedition to Alaska was "unfinished business"

Living on tundra berries and in primitive conditions, he began work on his "unfinished business".

And eventually he struck gold. He found the body of a woman - whom he named Lucy.

The body was well preserved because Lucy had been very fat and her body was therefore slow to decay.

His expedition cost �1,940. Dr Taubenberger said his contribution had been "amazing".

The research into the virus is still continuing and scientists hope they will be able to understand how a virus becomes a pandemic before the next one comes around.