The Spanish flu killed at least 50 million people worldwide between 1918 and 1919, with some estimates reaching as high as 100 million. In the United States alone, roughly 675,000 people died. At the time, the world’s population was about 1.8 billion, meaning the pandemic wiped out somewhere between 2.5% and 5% of all people on Earth in just over a year.
Why the Death Toll Is Still an Estimate
The “at least 50 million” figure is the most commonly cited number, used by the CDC and most public health institutions. But reliable record-keeping was limited or nonexistent across much of Asia, Africa, and South America in 1918, so the true number is almost certainly higher. Researchers who have tried to account for gaps in the data have pushed estimates to 100 million. The range is wide because there is no way to close it with certainty a century later.
Three Waves, One Deadly Peak
The pandemic arrived in three distinct waves. The first, in spring 1918, spread widely but was relatively mild. The second wave, from September to November 1918, was catastrophic. It was responsible for the majority of all deaths attributed to the pandemic. In the United States, October 1918 alone killed an estimated 195,000 Americans. A third wave followed in the winter and spring of 1919, adding still more deaths but at a lower intensity than the autumn surge.
The speed of the second wave was staggering. Communities that seemed untouched in the spring were overwhelmed within weeks once the fall wave arrived. Cities that implemented social distancing measures early, like closing schools and banning public gatherings, fared measurably better than those that delayed.
The Unusual Age Pattern
Most flu pandemics kill the very young and the very old, producing a U-shaped curve when you plot death rates by age. The 1918 pandemic added an alarming third spike: adults between 20 and 40 years old died at extraordinary rates, creating what epidemiologists call a W-shaped mortality curve. Healthy young adults, the group you’d normally expect to weather the flu with ease, were among the most likely to die.
The leading explanation centers on the immune response itself. Strong, healthy immune systems may have overreacted to the virus, flooding the lungs with inflammatory cells and fluid. This made younger adults more vulnerable, not less. The excess deaths in this age group were closely tied to higher rates of secondary bacterial pneumonia, a complication that arose after the initial viral infection damaged the lungs.
Most Deaths Came From Bacterial Infection
The influenza virus itself was not the direct killer in most cases. Medical experts at the time, and modern researchers reviewing the evidence, agree that the majority of deaths resulted from secondary bacterial pneumonia. The virus attacked the respiratory tract and left it vulnerable to bacteria that would normally be harmless. Without antibiotics, which wouldn’t be available for another two decades, these bacterial infections were often fatal. Cases of rapid, hemorrhagic viral pneumonia did occur but were considered uncommon even during the pandemic’s worst stretches.
Where the Pandemic Hit Hardest
India suffered the single largest national death toll, losing at least 12 million people, roughly 2% of the population of British India at the time. Some provinces were hit far harder than others. The Central Provinces lost an estimated 5.7% of their population, while Bengal lost about 0.4%.
Indigenous and isolated communities experienced some of the most devastating per capita losses anywhere in the world. In Alaska, 51% of all deaths during 1918 and 1919 were caused by the flu, and Alaska Native communities bore 82% of those deaths. The village of Brevig Mission near Nome lost nearly its entire population: only 8 of 80 residents survived. On a per capita basis, Alaska and Samoa suffered among the worst death rates of any place on Earth.
How the Spanish Flu Compares to COVID-19
The Spanish flu remains the deadliest pandemic in modern history. Its estimated toll of 50 to 100 million deaths occurred in a world with less than a quarter of today’s population, which makes the proportional impact far greater than anything since. COVID-19, by comparison, caused an estimated 27 million excess deaths between January 2020 and November 2023. In absolute numbers, COVID-19’s toll was significant, but as a share of the global population, the 1918 pandemic was in a different category entirely.
The difference reflects both the biology of the viruses and the tools available to fight them. In 1918, there were no antivirals, no antibiotics for secondary infections, no mechanical ventilators, and no vaccines. Public health responses relied entirely on non-pharmaceutical measures like quarantines and mask mandates, applied unevenly and often too late.