The influenza epidemic of 1918-19 killed an estimated 550,000 American, a ghastly toll, but one that, compared to the rest of the world, seems almost good fortune. New research places global mortality from the pandemic at 30 million people. Previously, it was estimated at 21.5 million. In either case, it ranks as the 20th century's most readily forgotten global disaster, and almost certainly the deadliest epidemic in recorded history. Even today, its virulence remains an utter mystery. The disease swept whole countries in weeks.

An entire port city in Nigeria was infected by fewer than 10 persons. Only American Samoa and portions of northern and eastern Iceland were able to establish successful quarantines. Though mortality was high in all age groups, it was highest in people 20 to 40 years old, exactly the population that usually would be the hardiest. "I did a history of the influenza epidemics in the 18th and 19th centuries, and one of the things I looked for was anything that resembled this age pattern," says K. David Patterson, a professor of history at the University of North Carolina in Charlotte. "I found nothing like it. It is unique." Patterson and colleague Gerald F. Pyle published their recalculations of the epidemic's mortality last year in the Bulletin of the History of Medicine. According to the research, India probably had the highest mortality, between 42 and 67 deaths from influenza per 1,000 population. In contrast, the death rate in the United States was 5.2 per 1,000. Though the disease became known the "Spanish flu," the best evidence is that it broke out March 8, 1918, in Camp Funston, Kansas. Scientists speculate that it was the result of a dramatic genetic "shift" in the influenza virus.

Such events occur every 30 or 40 years and usually usher in a worldwide epidemic of the new strain. This particular strain, however, traveled faster and was more dangerous than its predecessors. It arrived in Europe on American troop ships in early April 1918, and by July it spread as far east as Poland.

What happened then -- if current theories are correct amounts to bad luck of almost incalculable proportions. Somewhere on the Western Front, the virus underwent a second "shift" and the result was a virus far more lethal than even its lethal parent. It was that strain that swept round the world, killing millions. In the spring of 1919, the virus played itself out, and disappeared. Though there are no samples of the pandemic's strain, antibody tests of the people who lived through it have given biologists a guess at the identity of the virus. They believe the virus, or a close descendant, is now "archived" in pigs, one of the many non-human reservoirs of influenza, and one where a virus changes much more slowly than it does in man. In 1976, an outbreak of influenza at Fort Dix, N.J., killed a healthy soldier. When laboratory studies showed his virus to be similar to the one in pigs, virologists feared the deadly 1918 strain had reentered the human population. This led to production of the controversial "swine flu" vaccine. As it happened, the Fort Dix strain never reappeared. There is good evidence, however, that there is an occasional exchange of virus between animal and human populations, and that these may be the source of some of the more virulent influenza epidemics.