History is supposed to teach. But past flu pandemics, it turns out, don't teach much about whether today's bird flu will become a human megakiller or just make some scientists and officials look like Chicken Little.

In a viral sense, the sky has fallen three times in the last century -- 1918, 1957 and 1968 -- when "superflu" strains killed millions more people than annual flu epidemics routinely do.

Back then, there weren't surveillance systems or modern genetic tools to detect and document viruses as they evolved into killer strains. Because scientists don't know how that evolution happened or how long it took, they can't tell us whether what we're seeing with now is the runup to a pandemic.

"My crystal ball doesn't allow me to answer that," said Dr. Frederick Hayden, a University of Virginia flu expert.

IDEA WE'RE DUE FOR KILLER STRAIN DEBUNKED

Leading scientists discount the notion that flu pandemics happen in regular intervals and that the world is overdue for a new one. They don't even agree on how bad it is that bird flu has spread to more types of birds. Instead of an appetite for people, the germ is showing a fondness for birds, some say.

They do agree on the need to make vaccine, stockpile drugs and be prepared.

"We have to run scared" and be glad if precautions prove unneeded, said Dr. Edwin Kilbourne, a microbiologist and flu virus expert at Cornell University.

Nobody can say when the next pandemic will emerge -- just ask Kilbourne. When "swine flu" appeared in 1976, after global flu epidemics in 1957 and 1968, he championed the idea that pandemics appear every 10 years or so. But swine flu didn't become a pandemic, and neither has anything else.

BUT MUTATIONS KEEP APPEARING

The first documented cases of bird flu in people occurred in 1997 in Hong Kong, where six died and the entire poultry population of about 1.5 million was slaughtered in three days to control the outbreak. The current flu virus strain, classified as H5N1 like the Hong Kong germ, appeared in people in 2003. More than 60 people in Asia have died.

Nearly all the human victims caught the virus from close contact with sick chickens, with only one confirmed case of a person infecting another person. The fear is that H5N1 will acquire the ability to spread easily from person to person, sparking a pandemic.

For that to occur, the virus "must make people sick and spread easily. Very few new influenza strains do those two things," said Dr. Dennis Maki, infectious diseases chief at the University of Wisconsin-Madison.

Michael Osterholm, a government flu adviser from the University of Minnesota School of Public Health, still sees big dangers. "There have been many, many mutations with this virus . . . and it only continues to march around the world" instead of abating, he said.

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