The Black Death, a bubonic plague pandemic that swept across Europe and Asia from 1347 to 1351, killed more people as a proportion of the global population than any other pandemic in recorded history. It wiped out at least 25 million Europeans alone, representing somewhere between 25% and 75% of the continent’s population. When deaths across Asia and the Middle East are included, estimates range from 75 million to 200 million, though precise figures from that era are impossible to pin down.
The Black Death: 1347 to 1351
The plague bacterium, spread primarily by fleas on rats, tore through medieval societies that had no understanding of germ theory and no effective treatment. Within four years, it reshaped the demographics of an entire continent. The sheer percentage of the population lost is what sets the Black Death apart from every pandemic that followed. Even at the conservative end, losing a quarter of Europe’s people in under five years is a scale of death unmatched before or since.
The disease didn’t end in 1351, either. Waves of plague returned for centuries, keeping populations suppressed. The demographic hole it created took Europe roughly 200 years to recover from.
The Plague of Justinian: An Earlier Catastrophe
Centuries before the Black Death, the same plague bacterium struck the Eastern Roman Empire in 541 CE. The Plague of Justinian hit Constantinople particularly hard. Eyewitness accounts describe officials who stopped counting bodies after reaching 230,000. Modern modeling of the outbreak in Constantinople estimates between 250,000 and 340,000 deaths in the city alone, roughly half to two-thirds of its population.
Across the broader empire over the following years, maximalist historians estimate between 15 and 100 million deaths, potentially killing a quarter to half the population of the Eastern Roman world. The enormous range in those estimates reflects how little reliable data survives from the 6th century. What’s clear is that the Justinianic Plague was devastating on a civilizational scale, weakening the Byzantine Empire at a critical moment in its history.
Smallpox in the Americas: 1500s
When Europeans arrived in the Americas, they brought diseases that Indigenous populations had never been exposed to. Smallpox was the deadliest of these. Within a single generation of European contact, the population of much of the Americas fell by an estimated 90%. In the Aztec Empire alone, between 2 and 15 million people died from smallpox between 1520 and 1527.
Whether this qualifies as a single “pandemic” is debatable, since smallpox spread across the Americas in overlapping waves over decades rather than in one defined outbreak. But the cumulative toll was staggering. The pre-contact Indigenous population of the Americas is estimated in the tens of millions, and disease (more than warfare or famine) was the primary driver of its collapse. No other disease has so thoroughly reshaped an entire hemisphere’s population.
The 1918 Influenza Pandemic
The Spanish Flu remains the deadliest pandemic of the modern era. Early estimates placed the global death toll at around 21.6 million. That figure has been revised upward repeatedly as researchers have examined more data. A widely cited 2002 analysis suggested 50 million deaths as a baseline, with 100 million as an upper bound.
More recent reassessments have pushed back on the highest numbers. One analysis found that simulations producing more than 25 million deaths were difficult to support using verified mortality records from the period. The realistic range likely falls between 25 and 50 million, representing 1% to 3% of the world’s population at the time. The virus killed in three distinct waves over roughly two years, with the second wave in the fall of 1918 being by far the most lethal. Unusually, it hit young, healthy adults hardest rather than the very old and very young.
HIV/AIDS: A Slow-Burning Catastrophe
Unlike the fast-moving pandemics above, HIV/AIDS has killed on a timeline measured in decades rather than years. Since the epidemic began in the early 1980s, approximately 44.1 million people have died from AIDS-related illnesses worldwide. That cumulative toll rivals or exceeds the Spanish Flu, making HIV one of the deadliest infectious disease events in human history.
The key difference is speed. The Black Death killed tens of millions in under five years. HIV has accumulated its death toll over more than 40 years, with the worst mortality concentrated in sub-Saharan Africa during the 1990s and early 2000s before antiretroviral treatment became widely available.
COVID-19 in Context
By the end of 2020, roughly 1.8 million COVID-19 deaths had been officially reported worldwide. But the WHO estimated that actual excess deaths (the number of people who died above what would normally be expected) were at least 3 million for 2020 alone. By mid-2021, official reports had passed 3.4 million, with excess mortality likely far higher. Cumulative estimates through the full course of the pandemic place excess deaths in the range of 15 to 25 million globally.
In raw numbers, that puts COVID-19 in the same general range as some historical pandemics. As a percentage of the global population, though, its toll was far smaller, reflecting advances in medicine, public health infrastructure, and the rapid development of vaccines.
Why the Numbers Are Always Ranges
Every historical death toll you encounter is an estimate, and often a rough one. For medieval pandemics, records were sparse, inconsistent, and limited to certain regions. Even for modern events, factors like non-uniform definitions of what counts as a pandemic death, limited testing, and inconsistent reporting across countries make precise figures elusive. Excess mortality (comparing total deaths in a given period to what would be expected in a normal year) is the most reliable method, but it requires good baseline data that many countries simply don’t have.
This is why the Black Death’s toll is expressed as a range spanning tens of millions, and why estimates for the 1918 flu have shifted from 21 million to potentially 100 million over the past century. The ranking of history’s deadliest pandemics depends heavily on which estimates you use and whether you’re measuring raw deaths or the percentage of the population lost.
Raw Deaths vs. Population Share
The answer to “which pandemic killed the most people” changes depending on how you measure it. By sheer body count, the Black Death and HIV/AIDS compete for the top spot, each in the range of 25 to 50 million or more. By percentage of the affected population, nothing comes close to what happened in the Americas after European contact, where 90% of Indigenous peoples were killed by diseases they had no immunity to. And if you’re asking which pandemic most thoroughly disrupted an entire civilization’s trajectory, the Black Death and the Plague of Justinian both reshaped the political and economic order of their eras in ways that took centuries to recover from.
The Black Death is the most common answer for good reason. It combined an extraordinarily high fatality rate with rapid, widespread transmission across the most populous regions of the medieval world, killing a larger share of the global population in a shorter time than any other single outbreak.