History is supposed to teach lessons. Past flu pandemics, it turns out, don't teach much about whether today's bird flu will become a human killer 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 - in 1918, 1957 and 1968 - when "super-flu" strains killed millions more people than annual flu epidemics.

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 are seeing with bird flu now is the beginnings of a pandemic or a near miss.

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

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 growing 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.