It Struck the Young

Most flu strains kill the very old and the very young. The 1918 virus broke that pattern — it was strikingly deadly for adults in the prime of life, roughly ages 20 to 40.

Why this age group suffered so badly is still debated. One proposed explanation is that an overreacting immune response, a so-called cytokine storm, may have flooded the lungs in some cases. But the strongest evidence points to secondary bacterial pneumonia as the leading cause of death, and a 2023 study of victims' remains found that frailer people, not the healthiest, were the most likely to die.

Either way, the toll was severe in crowded conditions. In the U.S. Army, about 45,000 soldiers died of influenza and pneumonia — nearly as many as the roughly 53,000 killed in combat, so that more American soldiers died of disease than of enemy fire.