"Most bird flus emerge briefly and are relatively localized," said Dr. Andrew T. Pavia, chief of the division of pediatric infectious diseases at the University of Utah and chairman of the pandemic influenza task force of the Infectious Diseases Society of America. The most worrisome thing about H5N1, he said, is that it has not gone away.

Some scientists suspect that if H5N1 has not caused a pandemic by now, then it will not, because it must be incapable of making the needed changes. But others say there is no way to tell what the virus will do as time goes on. And they point out that no one knows how long it took for the 1918 virus to develop the properties that led to a pandemic.

Meanwhile, H5N1 seems to be finding its way into more and more species. Once known to infect chickens, ducks and the occasional person, the virus is now found in a wide range of birds, and it has infected cats.

"It killed tigers at the Bangkok zoo, which is quite remarkable because flu is not traditionally a big problem for cats," Pavia said.

It has also infected pigs, which in the past have been a vehicle to carry viruses from birds to humans.

"We should be worried but not panicked, Pavia concluded.

The timing of the bird flu's emergence also makes scientists nervous, because many believe that based on history, the world is overdue for a pandemic. Pandemics occur when a flu virus changes so markedly from previous strains that people have no immunity, and vast numbers fall ill.

"In the 20th century there were three pandemics, which means an average of one every 30 years," Fauci said. "The last one was in 1968, so it's 37 years. Just on the basis of evolution, of how things go, we're overdue."

Dr. Bruce Gellin, director of the National Vaccine Program Office, said: "You get this sense of compounding risks. First, it's in some birds. Then more. Then more area, then more mammals and then to humans, albeit inefficiently."

In just a few instances, Gellin noted, the virus does appear to have spread from person to person.

"The only thing it hasn't done is to become an efficient transmitter among humans," he said. "It's done all the other things that are steps toward becoming a pandemic virus."

But not everyone is equally worried about the bird flu.

The fear "is very much overdone, in my opinion," said Dr. Edwin Kilbourne, an emeritus professor of immunology at New York Medical College, who has treated flu patients since the 1957 pandemic and has studied the 1918 flu.

The bird flu, he said, is distantly related to earlier flus, and humans have already been exposed to them, providing some resistance.

Scientists also say that the death rate may not be as high as it appears, because there may be some milder cases that have gone unreported.