The experts, it said, were "keeping their lips sealed and their fingers crossed."

But neither silence nor prayer could prevent the 20th century's second major flu pandemic from sweeping across the globe.

By the spring of 1958, what came to be known as Asian flu had claimed two million lives and reminded scientists how little they really knew about one of Man's most lethal, mutating predators.

Even today, half a century later, there are lessons to be gleaned from that outbreak as the world warily eyes H5N1 bird flu, wondering if — or when — it will mutate into a form that spreads easily among humans.

"1957 was the first pandemic in the modern era of virology," explained David M. Morens, a medical epidemiologist at the National Institute of Allergy and Infectious Diseases in the United States. "One thing we learned is that pandemic viruses can be the descendents of previous ones."

Born in the southern Chinese province of Guizhou in late February 1957, the H2N2 virus of Asian flu at first travelled south, east and southeast, taking two or three months to reach the fringes of Asia.

A virus can get anywhere

Edward Kilbourne, emeritus professor of microbiology and immunology at New York Medical College, remembers 1957 as if it were yesterday.

"I was working at Cornell New York Hospital when the first patients were admitted — they had very serious symptoms and there were some fatalities," he recalled in an interview.

In performing autopsies on the victims, he and his colleagues noticed that the lungs bore a striking resemblance to descriptions of those infected during horrific Spanish flu of 1918, which by some estimates slew 50 million people.

"It was significant because it was the second pandemic of the 20th century, and because it could kill people outright," said Kilbourne, who went on to become a leading authority on influenza.

Most flu fatalities stem from underlying medical conditions, typically heart conditions, or secondary infections such as bacterial pneumonia. In 1957, women in the third trimester of pregnancy were also especially vulnerable.

Like the 1918 strain, Asian flu originated in an avian virus. But in a critical difference that was not understood until very recently, it found another way to infect human beings.

In 2005, scientists sequenced the 1918 influenza "A" virus, recovered from tissues frozen for more than eight decades in Alaskan permafrost.

What they discovered was both startling and alarming — the strain, their research indicated, had jumped directly from a bird flu and adapted itself to a human host, spreading like wildfire from there.

By contrast, Asian Flu, as well as the century's third and last pandemic in 1968, emerged from a process called reassortment in which two strains merge to form a third. In this case, it was a descendant of the 1918 strain — known as H1N1 — and an avian flu, H2N2.

Another key difference between 1918 and 1957, experts say, is the fatality rate: Spanish flu killed approximately one out of every 40 people it infected.

"The mortality of the Asian flu pandemic never rose that high — even at its peak it never exceeded one percent," commented George Brownlee, a molecular biologist at the William Dunn School for Pathology at Oxford University.

"We simply do not understand why the Spanish flu had such a high mortality rate."

The 1957 flu provided some sobering lessons for today.

Among them: the need to swiftly identify an outbreak and contain it by isolating patients, quarantining those in contact with them and establishing border checks people with fever or other flu symptoms.

Once the pathogen entered the mainstream population, "it spread very quickly," recalls Kilbourne, noting that school children were the main vehicles of transmission in the United States. "It spread even in the summer, when conditions are less favorable."

Instead of taking months to cross an ocean, a flu virus these days would take hours, hiking a ride with an infected corporate executive, vacationing family or backpacker.

"In 1918 — before the era of commercial air travel — the major fatal wave of flu moved all over the world very quickly," cautioned Morens. "A highly-transmissible virus can get anywhere even without modern infrastructure."

Stacked in our favour is the power of the Internet to inform, and the power of genetics and Big Pharma to identify a new killer virus and devise a vaccine against it.

But making a vaccine quickly and distributing it to the people who need it most are tasks as relevant today as in 1957. Then, vaccine production hit its stride after the flu peaked, and very few vaccines were distributed in poor countries.

AFP