The reemergence of a deadly strain of bird flu in Asia has spurred jitters among public health officials who fear the conditions are ripe for a worldwide pandemic.

"Pandemics come in cycles and we´re overdue for one," said Dr. Donald Perlman, an immunologist at the University of Medicine and Dentistry of New Jersey.

"If this avian flu gets loose in the world, it will be a major concern," he said. "It could be a huge disaster."

Those concerns were heightened last week when Thai health officials revealed that a 26-year-old woman may have caught the virulent H5N1 strain of avian influenza from another person, and not a bird - which is how all the other victims have been infected this year.

Virus samples taken from the Thai victim are being studied at the federal Centers for Disease Control and Prevention in Atlanta to see if it has mutated and gained the ability to jump from person to person.

"That´s a big nightmare that we have," said Dr. Niranjan Bhat, an epidemiologist with the agency´s influenza branch. "The events in southeast Asia have really increased the concern."

Most flu epidemics in people fizzle out because a good deal of the population has an existing immunity to the strain. Still, about 36,000 people in the U.S. - mostly the elderly and already ill - die of flu-related causes every year.

But when a novel flu strain jumps the species barrier, it can kill even the healthiest of people because they don´t have any immunity to the disease, explained Dr. Edwin Kilbourne, an emeritus professor at New York Medical College in Valhalla, N.Y.

Kilbourne extensively studied the disastrous Spanish flu pandemic of 1918. That worldwide outbreak killed between 20 million and 100 million people, many in the prime of their lives. Recent research suggests that the Spanish flu also may have begun as an avian flu strain.

"It´s almost a given that if you have a pandemic, it comes from outside human sources," Kilbourne said.

However, just because a virus jumps the species barrier doesn´t mean it has the ability to jump from person to person.

But the H5N1 virus also has been found in pigs, which dramatically increases the odds that the virus will pick up the ability to be transmitted from person to person.

"Pigs can be easily infected by both avian strains and human strains," CDC´s Bhat said. "So they can serve as mixing vessels."

This concern prompted federal health czar Tommy Thompson to adopt a pandemic influenza preparedness and response plan recently and award a contract last week for 2 million doses of an experimental avian flu vaccine.

And as was shown by last year´s outbreak of SARS - severe acute respiratory syndrome - international travel has made it much harder to contain epidemics, said Dr. Martin Blaser, an infectious disease expert at NYU Medical Center.

Although previous flu pandemic worked their way slowly from port cities, the Hong Kong flu of 1968 "arrived almost simultaneously everywhere because of airplane travel."