Bird flu fear has gripped the globe, as the disease spreads west from its origins in East Asia.  Public health officials around the world are raising the specter that the virus could set off an international influenza pandemic, if it mutates into a form easily passed among people. 

There are 15 strains of the avian flu virus, but the most virulent around is the one known by its scientific code name H5N1.  It has sickened and killed countless millions of birds on the Eurasian continent, and has caused more than 60 human deaths in Hong Kong and Southeast Asia since 1997.

Once a person is infected, its impact on the lungs is nasty, as University of Maryland flu researcher Daniel Perez explains.

"The symptoms will basically start like any flu symptoms," he said.  "But, then, instead of slowing down like a normal flu infection in a normal adult, it will go basically into the extreme of causing viral pneumonia and death."

Nearly all birds that get infected die.  The human death rate has been lower, but physician Gregory Poland of the Mayo Clinic College of Medicine in Minnesota says it is far higher than the deadly 1918 avian flu pandemic.

"In 1918, that virus killed about two-percent of all the people that it infected, leading to an estimated 40 to 60 million deaths," said Mr. Poland.  "The current avian influenza strain circulating is killing 50 percent, not two percent, but 50 percent of the people that it has infected.  If that virus gains the ability to easily transmit from human-to-human, it will be a disaster of unprecedented proportion."

Public health experts worry that it is close to doing so. It mutates rapidly, and quickly acquires genes from viruses infecting other animals.  Dr. Poland says, this is what allows it to adapt so easily to other species, as it has to pigs and cats, and what could set off a human pandemic.

"It's sort of like a slithering, writhing mass of viruses, all interacting with one another and exchanging genetic material, learning how to adapt to the host that it infects," added Dr. Poland.

A recent study by U.S. researchers shows that bird flu might be able to skip this mixing step before adapting to people.  They found that it has several of the same genetic mutations as the highly deadly 1918 virus, which they reconstructed from samples extracted at the time from cadavers. 

Researcher Jeffrey Taubenberger, of the Armed Forces Pathology Institute, says these mutations might help the virus adapt to people, without having to combine with a human strain.

"We now think that the 1918 virus was an entirely avian-like virus that adapted to humans," he explained.  "This is a different situation than the last two pandemics we had, which are mixtures, in which a human-adapted influenza virus acquired two or three new genes from an avian influenza source. So, it suggests that pandemics can form in more than one way, and this is a very important point."

The H5N1 bird flu might have come at the right time for it to prosper.  The physician who directs infectious disease research for the U.S. government, Anthony Fauci, says about three flu pandemics occur each century, and the time is right for another one.

"We have not had one since 1968, so you are talking now, 37 years we have not had one," he noted.  "That is what we mean when we say, from a pure evolutionary standpoint, we are generally due.  Is it going to be this H5N1?  I do not know, but there are enough circumstances going on now that makes this a real threat."

A lack of medical defenses worsens the situation.  A recent study from the U.S. government's Centers for Disease Control (CDC) shows that worldwide resistance to influenza drugs has increased by 12 percent in the last decade.  H5N1 is resistant to one of the two major classes of flu drugs, amantadine, and scientists report signs in Asia it is learning to defy the other one, Tamiflu.

Moreover, no one knows if existing flu vaccines will work against H5N1, and a vaccine specific to it is only in the testing stage. In addition, global vaccine manufacturing capacity is low.

So, the University of Maryland's Daniel Perez says, the slaughter of infected birds remains a crucial protective measure.

"Right now, that is the only tool that is effective in controlling avian influenza," added Mr. Perez.