(FORTUNE Magazine) – YOU CAN'T LOOK INTO BIRD FLU WITHOUT HEARING that we're "overdue" for a pandemic potentially as bad as the lethal Spanish flu of 1918. Given the deadliness of the H5N1 flu strain now widespread in Asian birds--it has killed some 70% of the several dozen people it's infected--the dismaying claim seems quite reasonable. But maybe it's exaggerated.

From a flu virus's point of view, the winter of 1918 was heaven on earth. Never was it easier to infect so many people so fast. Picture myriad Europe-bound U.S. recruits in World War I coughing around stoves in flimsy barracks; rapid movement of thousands of infected troops in every direction; no far-flung organizations monitoring the virus's spread; no flu diagnostics, vaccines, or antiviral drugs; and no antibiotics to quell secondary bacterial infections, the main cause of flu death. No one even knew the cause, which wasn't nailed down until the 1930s, when a runny-nosed ferret sneezed in a British researcher's face, giving him flu and paving the way for animal studies that revealed influenza's viral origin.

Today's global jet travel does enable flu viruses to get from A to B fast. But when they get to B, they don't usually land in jam-packed wartime hospitals where within hours they can jump from one person to hundreds or thousands, as they did in 1918. The population mixing favored by modern world travel may also have hurt flu viruses' cause: It means that most of us have been exposed to more flu strains than people had been in 1918, says Walter Dowdle, former deputy director of the Centers for Disease Control and Prevention. "There's a huge amount of generic flu resistance out there," says Dowdle, now at the nonprofit Task Force for Child Survival and Development in Atlanta. Such resistance may partly explain why the flu pandemic of 1957 was much less lethal than 1918's, and the one of 1968 was milder yet.

The fact that the H5N1 virus has spread widely in birds is worrisome, for by mixing with other flu viruses present in avian cells, H5N1 may acquire genes for readily spreading in people--a talent it now lacks. But experts who assert that H5N1 is steadily progressing toward pandemic form are going too far, says Richard Schabas, former chief medical officer of Ontario, who's now at York Central Hospital in Richmond Hill, Ontario. "We have no idea why some avian flu strains leap the species boundary to spread in humans," he explains. "For all we know, there may be hundreds of viruses that have gone down this route [of infecting birds as H5N1 has] and didn't make the grade" as human strains. If so, the odds that H5N1 will mutate into a Spanish flu look-alike may be much smaller than has been suggested by all the alarming headlines about it.

Indeed, says Schabas, "every year we get told the new plague is about to arrive. A few years ago it was hantavirus, then Ebola, then West Nile virus, then SARS. And every year it doesn't. Infectious-disease experts have learned that they can get attention for their cause by talking about exotic new diseases, not things like hand washing or wound sterility. I'm very concerned that by crying wolf, they're in danger of losing credibility and getting resources put into the wrong things. TB, malaria, and HIV aren't sexy new diseases, but each year they kill millions of people.

"To the best of our knowledge, influenza pandemics are rare, random events. You're never 'overdue' for a random event." To say otherwise "is the kind of thinking that keeps Las Vegas in business." -- D.S.