In seeking to understand the remarkably deadly 1918 flu pandemic— which killed at least 20 million people around the planet— scientists have focused on the influenza virus, combing its genes for clues to its malevolence. But demographer Andrew Noymer of the University of California at Berkeley thinks people are overlooking a second culprit: Mycobacterium tuberculosis, the tuberculosis bacterium.
Noymer reached his iconoclastic conclusion after poring over acres of data on 20th-century death rates in the United States. One statistic stood out. The rate of deaths from TB plunged from 157 per 100,000 in 1918 to 103 per 100,000 in 1921, right after the flu pandemic. He found no similar decrease in mortality from other chronic ailments such as cancer. That pattern implies that many of those who died from the flu were already infected by TB. And he notes that M. tuberculosis carves out cavities in the lung. Those cavities would have been perfect breeding grounds for pneumonia, which finished off most flu victims in 1918.
Between one quarter and one half of all Americans felled by the flu were already infected with tuberculosis, Noymer estimates, which helps explain why so many who died were adults. Flu usually targets infants and the elderly, but TB was then more common in adults. If Noymer is correct, another similarly virulent flu epidemic here would not be so lethal today: "TB death rates are almost nil in the United States these days," he says. Much of the rest of the world is not so fortunate: "In developing countries— Africa, Latin America, parts of Asia— TB is still a major cause of death, so those countries certainly would be at risk."