Noymer's findings explain a peculiarity of the 1918 pandemic that killed at least 20 million people worldwide.
Normally, the influenza virus is not lethal to young and middle-aged people. Most of its victims are elderly. But in 1918, the typical victim was a man between the ages of 20 and 40, a group that normally has a very low death rate, said Noymer.
In the early 20th century, however, tuberculosis was a major killer of men in that age group, apparently because of transmission in factories where men worked in densely-packed, poorly-ventilated conditions, Noymer said. Men were about 30 percent more likely to die from TB than women were-a pattern closely paralleled during the flu epidemic.
In 1918, men were 35 percent more likely than women to die from flu. Of the 500,000 Americans who died that year, 280,000-300,000 were men.
"This can't be a coincidence," said Noymer. "I think TB is the missing piece of the puzzle. It explains why younger people, especially men, died in such great numbers. Scientists since 1918 have been searching for clues for why the 1918 epidemic was so deadly, especially in middle age. But people did not look at what happened to tuberculosis death rates, not only in the epidemic year, but in the years afterwards."
His findings explain another mystery. Scientists who have attempted to study the gene sequence of the 1918 influenza virus have seen nothing out of the ordinary, nothing to explain the flu's virulence.
"Never before or since have we seen a flu epidemic that was so virulent," said Noymer. "The spread was extremely rapid, as was the development of the infection. Almost everyone who died was gone in two weeks.
"I do believe my finding explains most of the deadliness of the 1918 epidemic. It doesn't prove that, if another strain were to appear, that the U.S. population would be safe, but it strongly suggests that we would fare much better."
Noymer's analysis shows that the 500,000 people who died in 1918 were almost exactly the number who would have been in various stages of disease from TB. Using pre-1918 death rates, Noymer calculated that 500,000 more TB deaths would have occurred between 1918 and 1932 had there never been a flu epidemic.
As a result of the excess death among men in 1918, a healthier male population was left, said Noymer. For years afterward, the life expectancy of men, which usually lagged behind women by six years, moved up to more closely resemble the female pattern. It was this startling change that sparked Noymer's research, when he saw something no demographer had ever noticed before - a precipitous drop in 1919 in the gender differential from six to two years.
"When I saw that," said Noymer, " I said to myself, 'That's the flu!' And, surprise, surprise, it leaves the same mortality patterns on age and sex that TB does."
Co-author on the article is Michel Garenne, senior researcher at the French center for population and development studies, CEPED, Centre français sur la population et le développement.
