1918 influenza pandemic (Spanish flu)

In the closing months of the First World War and the two years that followed, an unusually lethal strain of influenza swept the globe. Caused by an H1N1 influenza A virus, the pandemic of 1918–1920 infected an estimated 500 million people, around a third of humanity, and killed somewhere between 17 million and 50 million, with some estimates reaching 100 million. It is often called the "Spanish flu", a misleading name that reflects wartime censorship rather than where the disease began.

Newspapers in the countries at war were censored to protect morale, but neutral Spain reported its outbreak openly from late May 1918. The country therefore looked like the centre of the epidemic, and by June The Times in London noted that everyone was calling it the "Spanish" influenza. Spanish officials protested that the disease had not been born in their country. Other nations had their own labels, often blaming foreigners, and the World Health Organization now lists "Spanish flu" as an example of a disease name to avoid.

Where the virus first emerged remains unknown. The earliest probable cases were recorded in Kansas in early 1918, and the pandemic is conventionally dated from 4 March 1918, when an army cook fell ill at Camp Funston; within days hundreds of soldiers there were sick. Other researchers point to outbreaks of a similar illness at British army camps at Étaples in France and Aldershot in England in 1916–1917, to a possible origin in Austria, or to northern China. A 2018 study found the evidence leaned towards a North American source but was not conclusive.

The first wave, in spring 1918, followed troop movements from American camps to the Western Front and on through Europe, North Africa, India and Japan. It was relatively mild, though it sickened large parts of the French, British and German armies. A far deadlier second wave began in late August 1918, spreading by ship from Brest in France to Boston and to Freetown in Sierra Leone, and from there across the Americas, Africa and Asia. October 1918 was the deadliest month of the pandemic. A parade in Philadelphia on 28 September to sell war bonds was followed by an outbreak blamed for 12,000 deaths, and celebrations of the Armistice on 11 November fuelled new outbreaks elsewhere. Further waves struck many regions in 1919 and early 1920.

Unusually, the virus killed many young adults: in the United States nearly half of pandemic flu deaths in 1918–1919 were among people aged 20 to 40. Most victims died of bacterial pneumonia that followed the flu, made worse by wartime malnutrition and crowded camps and hospitals. India suffered most in absolute terms, with perhaps 12 to 17 million deaths, about 5 per cent of its population. In the Dutch East Indies an estimated 1.5 million died, and in Western Samoa, then occupied by New Zealand, 22 per cent of the population died within two months.

No vaccines or antiviral drugs existed, and reporting systems did not usually track influenza, so responses were often late. Authorities closed schools, theatres and churches, limited public transport and in some cities required masks. Strict maritime quarantines kept the disease out of Australia until early 1919 and spared American Samoa and New Caledonia any deaths during the pandemic. Later research found that cities which acted early, and did not lift restrictions too soon, generally fared better.

At the time the pandemic was partly overshadowed by the war, whose dead were also mostly young adults. Scientists announced in 2005 that they had reconstructed the virus's genetic sequence from preserved tissue, including samples from a victim buried in the Alaskan permafrost. During the COVID-19 pandemic many commentators drew comparisons with 1918, from opposition to masks to the spread of misinformation.

Our 1914 and 1920 maps show the world before and after the war in which the pandemic spread, including British India, the region with the heaviest death toll.