The 1918 flu killed more people than the First World War

The influenza pandemic that swept the globe in 1918 is among the deadliest disease outbreaks in recorded history. It is estimated to have infected about 500 million people — one-third of the world’s population — and to have killed at least 50 million, including roughly 675,000 in the United States.

Its usual name is a slander. “Spanish flu” stuck only because of the First World War: combatant nations censored news of the outbreak to protect morale, while neutral Spain reported the spreading sickness freely. The disease looked Spanish simply because Spain admitted it. Its true origin is still debated — candidates include a U.S. Army camp in Kansas and northern France — and remains genuinely unknown.

The virus was also strangely cruel to the strong. Plotting deaths by age produces a W-shaped curve: high mortality among the very young and the old, as usual, but a deadly third peak among healthy 20-to-40-year-olds. The leading explanation is that a vigorous immune system overreacted, triggering a cytokine storm that flooded the lungs — the body’s own defenses turning lethal.

Unusually, the virus hit healthy young adults especially hard, alongside the very young and the elderly.

The autumn 1918 second wave proved far deadlier than the mild spring first wave, killing within days. With no vaccine and no antibiotics, control rested on isolation, quarantine, and closing public gatherings. The culprit itself, an H1N1 virus, was only sequenced in 2005 from preserved and exhumed tissue — finally putting a genome to the century’s worst pandemic.