Kilbourne and other experts also noted that when viruses become more transmissible, they almost always become less lethal. Viruses that let their hosts stay alive and pass the disease on to others, he explained, have a better chance of spreading than do strains that kill off their hosts quickly.

Moreover, he said, while much has been made of comparisons between the current avian flu and the 1918 strain, the factors that helped increase the flu's virulence in 1918 — the crowding together of millions of World War I troops in ships, barracks, trenches and hospitals — generally do not exist today for humans.

But an essential difference is that people carrying the flu today can board international flights and carry the disease around the world in a matter of hours.

Kilbourne emphasized that medical care had improved greatly since 1918. Although some flu victims then turned blue overnight and drowned from fluid leaking into their lungs, many more died of what are now believed to be bacterial infections, which can be treated with antibiotics.

Although the death toll from that flu was high, the actual death rate was less than 5 percent.

Also more people now live in cities, where they have probably caught more flus, giving them immunity to later ones. "In 1918, you had a lot of farm boys getting their first contact with city folks who'd had these things," Kilbourne said.

What researchers wish they could do now is look at a flu virus like H5N1 and predict whether it is heading down the genetic road to becoming a pandemic strain.

"I hope in the future we will be able to do that, work out which mutations are critical," Taubenberger said. "We know the 1918 strain had everything it needed."

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Contributing: Andrew Pollack, Donald G. McNeil Jr.