The current strain of bird flu concerning the world, showed up in Hong Kong eight years ago.  But avian influenza has a much longer history, including the strain that mutated into the deadly 1918 human flu pandemic.

When birds catch the flu, it is usually a mild form that barely ruffles their feathers.  But every once in a while, a virulent flu hits them, like the strain spreading worldwide that scientists call H5N1.  Virulent avian flu was first recognized in Italy in 1878 and is extremely contagious among birds and rapidly fatal, with death rates approaching 100 percent.

The virulent form has been seen about 100 times since then, but it had never been linked to humans until the 1997 Hong Kong outbreak.  That was the first time public health officials were aware bird flu could jump to people.  The cause is believed to be contact with infected fowl.

The man who directs infectious disease research for the U.S. government is Anthony Fauci.

"The first H5N1 that we had experience with was in 1997 in Hong Kong," he said.  "There were 18 cases and six deaths.  The Hong Kong authorities appropriately culled virtually all the chickens in Hong Kong, which put a real stop to that in 1997.  Unfortunately, it percolated along and came back in 2003, 2004, and 2005."

Bird flu can spread through international trade in live poultry.  Migrating birds also extend its reach, particularly waterfowl.

"Now, with the migratory fowl that are able to go long distances and cross-contaminate flocks in a number of different countries, it becomes even more problematic," he added.

By 2003, the H5N1 strain reappeared in South Korea, although the geographical epicenter soon became Southeast Asia, mainly Vietnam, but also Thailand, Cambodia, and Indonesia.  There have been almost 120 confirmed human cases in those four countries, 60 of whom have died a at 50 percent death rate.

The disease has spread westward beyond East Asia, but it has caused no human deaths outside of Asia.

Nor have there been any confirmed cases of what health officials fear most, bird flu transmission between people.  Physician Gregory Poland is a vaccine researcher at the Mayo Clinic College of Medicinel in Minnesota.

"It now kills its primary host, the bird," explained Mr. Poland.  "It has expanded its range into tigers and domestic cats.  It has directly infected humans.  The only thing it has not yet learned to do is efficiently transfer from human to human."

Recent medical studies suggest person-to-person transmission might already have begun.  A February New England Journal of Medicine report said an 11-year-old girl died in Thailand after exposure to a sick chicken, while her mother and aunt, who had no such bird exposure, also became ill, suggesting that human transmission occurred.  The mother also died.

Then in an October paper in the journal Nature, scientists say a Vietnamese teenage girl who recovered from bird flu might have caught it from her brother.

The president of the vaccine division of the U.S. pharmaceutical company Merck, Adel Mahmoud, says these are ominous signals of the virus's potential to trigger a global flu outbreak.

"That is just a very, very serious warning sign that viruses are recombining, moving from avian to animals to humans and then being transmitted within the human population," he noted.

As for the likelihood this will occur on a massive scale and begin a new pandemic, one has only to look back to 1918.  That was the start of an outbreak of a very lethal flu that killed an estimated 40 million to 60 million people worldwide within two years.  Examination of the genes of stored virus samples has since shown that it, too, originated in birds and did gain the ability to jump from person to person very quickly.

Survivor Andrew Jakomas remembers the result.

"We had little caskets for the little babies that stretched for four and five blocks, eight high, 10 high," said Mr. Jakomas.

If the bird flu virus adapts to human transmission and retains its virulence, experts like Gregory Poland predict that the number of deaths could be staggering, perhaps beating the 1918 toll significantly.

"I can think of nothing in our history - no war, no world war, no hurricane, nothing - that would compare with the potential devastation that pandemic influenza could wreak," added Mr. Poland.