The 1918 influenza pandemic, commonly called the Spanish flu, killed at least 50 million people worldwide, with about 675,000 of those deaths in the United States. The true number may be significantly higher. Researchers have estimated the real toll could fall anywhere between 50 and 100 million, making it the deadliest pandemic in modern history.
How the Death Toll Was Calculated
Early estimates from the 1920s placed global deaths at roughly 21.5 million. That figure stood for decades until researchers began reexamining mortality records from countries where deaths had been severely undercounted. A 1991 study pushed the estimate to a range of 24.7 to 39.3 million. Then in 2002, historians Niall Johnson and Juergen Mueller published an influential reassessment in the Bulletin of the History of Medicine, concluding the toll was on the order of 50 million, and that even this number could be “substantially lower than the real toll, perhaps as much as 100 percent understated.”
The difficulty lies in gaps in record-keeping across much of Asia, Africa, and South America during 1918. Many countries had no reliable vital statistics systems, so researchers rely on indirect methods like comparing expected deaths in a given year to the actual number recorded. In regions with no records at all, estimates are extrapolated from neighboring countries with similar demographics.
Where Deaths Were Concentrated
India was by far the hardest-hit country. Estimates of Indian deaths range from 12 million to 18 million, depending on the study and methodology. A 1919 government report by the Sanitary Commissioner counted approximately 6 million deaths in India alone, but later analyses using census data and excess mortality calculations doubled or tripled that figure. No other country came close to this toll in absolute numbers.
In the United States, the estimated 675,000 deaths made the pandemic the deadliest event in American history at the time. For context, roughly 117,000 American soldiers died in World War I, which was still underway when the pandemic struck. The virus moved through military camps, cities, and rural communities with startling speed, overwhelming hospitals and morgues.
The Deadly Second Wave
The pandemic unfolded in three distinct waves, and they were not equally lethal. The first wave, in the spring of 1918, was relatively mild. People got sick, but mortality was low compared to what followed. The second wave, arriving in the fall of 1918, was catastrophic. It was responsible for the vast majority of deaths in the United States and much of the world. A third wave followed in the winter and spring of 1919, less severe than the second but still deadly.
What made the second wave so much worse remains a subject of study, but the virus appears to have mutated into a far more virulent form over the summer months. By September and October 1918, entire cities were shutting down. Philadelphia famously held a large parade on September 28 and saw its hospitals overwhelmed within days.
Why Young Adults Died at Unusual Rates
Typical seasonal flu kills mostly the very young and the very old, producing a U-shaped curve when deaths are plotted by age. The 1918 pandemic broke this pattern. It produced a W-shaped mortality curve, with a sharp spike among adults in their 20s and 30s. Peak mortality hit around age 30, while the elderly actually had lower-than-expected death rates compared to normal flu seasons.
This pattern has never been fully explained. One leading theory is that older adults had partial immunity from exposure to a similar flu strain decades earlier, while young adults had no such protection. Their strong immune systems may have also worked against them. The virus triggered an overwhelming immune response in the lungs, and the resulting inflammation caused massive damage. Most people who died did not die from the virus directly. Medical experts at the time, along with later research from the CDC, concluded that the majority of deaths resulted from secondary bacterial pneumonia, infections that set in after the virus had weakened the lungs and airways.
Scale Compared to Other Pandemics
The 1918 pandemic remains the deadliest outbreak of the modern era. The global population in 1918 was roughly 1.8 billion, meaning the virus killed somewhere between 2.5% and 5.5% of all people alive at the time.
For comparison, COVID-19 caused an estimated 27 million excess deaths between January 2020 and November 2023, according to Our World in Data. That’s a staggering number, but the world’s population had grown to nearly 8 billion by then, making the percentage far smaller. The 1918 flu killed a larger share of humanity in a shorter window of time, without antibiotics to treat bacterial complications, without ventilators, and without any understanding of viruses as the cause of influenza.