The Spanish flu killed at least 50 million people worldwide between 1918 and 1919, making it the deadliest influenza pandemic in recorded history. About 675,000 of those deaths occurred in the United States. The true global toll, however, remains debated, with estimates ranging from as low as 17 million to as high as 100 million depending on the study and methodology used.
Why Estimates Vary So Widely
For decades, the accepted death toll hovered around 20 million. That number was revised sharply upward in 2002 when researchers Niall Johnson and Juergen Mueller published a landmark study concluding that at least 50 million people died, and possibly as many as 100 million. Their work incorporated mortality data from countries that had been overlooked in earlier estimates, particularly in Asia and Africa, where record-keeping was sparse or nonexistent during the colonial era.
Not everyone agrees with the higher figure. An earlier study by Patterson and Pyle in 1991 estimated between 24.7 and 39.3 million deaths. More recently, a 2018 study by Spreeuwenberg and colleagues argued that earlier estimates were too high, arriving at a figure of 17.4 million. The CDC uses the “at least 50 million” figure, and that remains the most commonly cited number. The wide range reflects a simple reality: much of the world in 1918 had no reliable system for counting deaths.
The Deadliest Wave Hit in Autumn 1918
The pandemic arrived in three distinct waves. The first, in the spring of 1918, was relatively mild. The second wave, which struck in the fall, was catastrophic. In the United States, the virus returned through the port of Boston in September 1918, and within weeks it had spread across the country. Nearly 200,000 Americans died in October 1918 alone.
A third wave followed in the winter and spring of 1919, less deadly than the second but still significant. The overwhelming majority of total deaths came from that brutal autumn surge, when the virus spread rapidly through crowded military camps, cities, and rural communities simultaneously.
Young Adults Died at Unusual Rates
Most flu pandemics kill the very young and the very old, producing a U-shaped curve when you plot deaths by age. The Spanish flu did something different. It added a third peak in the middle, killing people between 20 and 40 years old at rates that had no precedent and have not been seen since. Researchers call this a W-shaped mortality curve.
The reason so many young adults died comes down to bacterial pneumonia. The flu virus damaged the lungs, and bacteria moved in to cause a secondary infection. In an era before antibiotics, that secondary pneumonia was frequently fatal. The unusually high rate of pneumonia cases in 20- to 40-year-olds, rather than some unique severity of the virus in that age group, drove the elevated death toll. Even children aged 5 to 14, who typically have the lowest flu mortality of any age group, died at roughly four times the rate seen in the previous major pandemic of 1889.
The Toll on Soldiers
World War I was raging as the pandemic peaked, and military camps became breeding grounds for the virus. Crowded barracks, constant troop movements, and the stress of wartime created ideal conditions for transmission. About 45,000 American soldiers died of influenza and related pneumonia by the end of 1918, nearly matching the 53,402 American combat deaths in the entire war.
Individual camps were devastated. At Camp Devens in Massachusetts, 757 soldiers died. At Camp Upton, 6,131 soldiers were hospitalized and 404 died before the outbreak was declared over on October 22. Camp Syracuse saw 2,289 flu cases out of a garrison of 12,000, with 208 soldiers dying. The virus did not respect the boundary between military and civilian life: 900 residents of the city of Syracuse died during the same outbreak.
How It Compares to Other Pandemics
The Spanish flu remains the deadliest pandemic of the modern era. At 50 to 100 million deaths, it killed far more people than World War I itself, which caused an estimated 20 million military and civilian deaths. The world population in 1918 was roughly 1.8 billion, meaning the pandemic killed somewhere between 1 and 5.5 percent of every person alive.
By comparison, the COVID-19 pandemic caused an estimated 27 million excess deaths between January 2020 and November 2023, out of a world population that had grown to nearly 8 billion. In raw numbers, the Spanish flu killed roughly twice as many people. As a share of the global population, the difference is far starker: the 1918 pandemic was many times more deadly relative to the number of people alive at the time.