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1918 flu pandemic
1918–1920· Event· Global
Well documentedSolid records, physical evidence or scientific consensus.
A deadly influenza pandemic in 1918–1920 that infected about a third of humanity and killed tens of millions.
See it on the History map →What happened
The virus, an H1N1 influenza strain, came in waves; the second, in autumn 1918, was the deadliest. Unusually, it killed many healthy young adults. Wartime censorship muffled news in the warring nations, but neutral Spain reported freely, so it was nicknamed 'Spanish flu' though it did not start there. There were no flu vaccines or antibiotics; masks, closures and quarantines were the main tools. Deaths are estimated at 17 to 50 million, and some estimates run higher.
Why it matters
It likely killed more people than the war itself and pushed governments toward modern public health, including disease tracking and national health ministries.
How we know
Death records and newspapers from 1918, and the virus itself: in 2005 scientists rebuilt its genome from victims' preserved lung tissue, including a body buried in Alaskan permafrost.
Still debated
Where it began (Kansas, France and China have all been proposed) and why it hit young adults so hard are still debated.
Connections (3)
▸World War I helped spread the 1918 flu pandemicMostly agreed
The connection
Crowded training camps, trenches and troop ships were ideal for spreading flu. Millions of soldiers moved between continents carrying the virus, and demobilization sent them home again. Wartime censorship played down the outbreak, and doctors and nurses were scarce because many were serving at the front.
When and how
Army camps such as Camp Funston, Kansas, and the British base at Étaples, France, in 1918; the deadly second wave struck Brest, Freetown and Boston almost together in August 1918.
How sure are we?
Military medical records document huge outbreaks in camps and on troopships; more US soldiers died of disease, mostly flu and pneumonia, than in battle.
Debates and open questions
Whether wartime conditions actually made the virus deadlier, for example by favoring virulent strains in the trenches, is a hypothesis, not proven.
▸1918 flu pandemic was the closest precedent for the COVID-19 pandemicMostly agreed
The connection
Before COVID-19, the 1918 flu was the benchmark for a modern pandemic: an airborne virus circling the world in waves. Its lessons, such as that US cities which closed schools and gatherings early had fewer deaths, shaped responses in 2020. Planners compared the two constantly, and COVID became the deadliest respiratory pandemic since 1918.
When and how
US pandemic plans of 2005–2007 drew on 1918 data; in 2020 officials in many countries cited 1918 when ordering lockdowns and mask rules.
How sure are we?
Studies comparing 1918 cities, notably St. Louis and Philadelphia, were cited in planning documents. 1918 killed an estimated 17 to 50 million; estimates vary.
Debates and open questions
Whether 1918's lessons fit a different virus, and whether lockdowns were worth their economic and social costs, remain debated.
▸HIV/AIDS pandemic has killed about as many as the 1918 flu pandemicMostly agreed
The connection
The 1918 flu killed tens of millions in about two years; HIV has killed about 42 million over four decades. The 1918 toll is so uncertain, roughly 17 to 50 million or more, that the two are broadly comparable. The difference is speed: HIV kills slowly over years, while the flu struck within days.
When and how
Worldwide, 1918–1920; HIV deaths counted by UNAIDS from 1981 to today.
How sure are we?
UNAIDS publishes yearly HIV death estimates; 1918 figures come from historical records and demographic modeling, with very wide ranges.
Debates and open questions
Which killed more depends on which 1918 estimate you use; older figures of up to 100 million are now considered too high by many researchers.