Historical Pandemic

1918 Influenza Pandemic

Also known as: Spanish Flu, Great Influenza

Death toll

50–100 million (3–5% of world population)

Period

1918–1920

Pathogen

Influenza A virus, H1N1 subtype

Transmission

Respiratory droplet

Overview

The 1918 influenza pandemic was the deadliest outbreak of infectious disease in the 20th century and one of the deadliest in recorded history. Caused by an H1N1 influenza A virus of avian origin, it infected roughly one-third of the world's population — around 500 million people — and killed between 50 and 100 million over about 15 months.

The pandemic was characterised by three waves (spring 1918, autumn 1918, winter 1919) and by an unusual mortality pattern: young adults aged 20–40 were disproportionately killed, likely because the virus triggered massive immune response in healthy adults (a "cytokine storm"). The autumn 1918 wave was by far the deadliest.

The popular name "Spanish flu" is a misnomer. The pandemic did not originate in Spain; Spain was simply the earliest country to report it freely because it was neutral in World War I and not subject to wartime press censorship. Likely origins include a Kansas military camp (Funston, March 1918) or northern France. Wartime troop movements accelerated transmission globally, and censorship of reports in belligerent countries delayed public-health response.

Geographic scope

Global; one-third of humanity infected

Peak year

1918

Timeline

March 1918
First recorded cases at Camp Funston, Kansas — spring wave begins.
May 1918
Spanish press freely reports outbreaks; "Spanish flu" name spreads.
August 1918
Second, much deadlier wave begins in Boston, Freetown, and Brest simultaneously.
October 1918
Peak monthly mortality; 195,000 deaths in the United States alone.
Winter 1918–19
Third wave; total US deaths exceed 675,000.
1920
Pandemic effectively ends; descendants of the 1918 H1N1 strain remain in circulation.
2005
Full 1918 virus genome reconstructed from preserved tissue samples.

Impact

The 1918 pandemic killed more people in a year than the First World War killed in four. Its unusual age distribution — targeting prime working-age adults — produced economic, demographic, and psychological effects distinct from typical respiratory epidemics. Public health capacity expanded in its aftermath, and the term "super-spreader" first entered epidemiological usage. Much of what is now considered standard pandemic response — quarantine, closure of public gatherings, mask mandates — was piloted or expanded during 1918–19. Cultural memory of the pandemic was long muted, overshadowed by the war.

How it ended

The pandemic ended as the virus mutated to less virulent forms and populations acquired immunity. The H1N1 lineage continued to circulate as seasonal flu until 1957, when it was replaced by H2N2 (the Asian flu). H1N1 returned in 1977 and again caused a pandemic in 2009.