An estimated 90 to 95 percent of the Indigenous population in the Americas died in the centuries following European contact, with infectious disease as the single largest cause. In raw numbers, that translates to tens of millions of deaths, though the exact figure depends on which pre-contact population estimate you start with. Scholars have debated that starting number for over a century, and the range is wide: from roughly 8 million to over 100 million people living across North and South America in 1492.
Why the Numbers Are So Uncertain
No census existed before Europeans arrived, so researchers have to work backward from archaeological evidence, early colonial records, and ecological modeling. The lowest credible estimates for the entire Western Hemisphere hover around 8 to 12 million. The highest, championed by demographers at UC Berkeley in the mid-20th century, exceed 100 million. A middle range that many modern scholars use falls between 40 and 60 million for the Americas as a whole, with roughly 5 to 15 million of those in what is now the contiguous United States and Canada.
Even the methods used to generate these numbers are contested. James Mooney, whose early estimates for North American tribes became a benchmark, consistently chose the minimum probable population for each group, according to later analysis of his unpublished notes at the Smithsonian. More recent scholars have argued those baseline figures should at least be doubled. A 2025 study published in the Proceedings of the National Academy of Sciences noted that population in what is now the continental United States may have already declined roughly 30 percent between 1150 and 1500 CE, before sustained European contact even began, likely due to climate shifts and regional conflict. That means even the “pre-contact” estimates may represent a population already below its peak.
If you take a moderate estimate of 50 to 60 million people in the Americas in 1492 and apply a 90 percent decline, the total loss reaches 45 to 54 million people over roughly 150 years. Disease accounts for the vast majority of that number, though it worked alongside warfare, enslavement, and displacement in ways that are impossible to fully separate.
What “Virgin-Soil Epidemics” Actually Means
The term describes what happens when a disease enters a population that has never been exposed to it. Every single person is susceptible. In Europe, diseases like smallpox and measles circulated constantly, killing some people (especially children) while leaving survivors with lifelong immunity. Over centuries, European populations developed a baseline of resistance. Indigenous peoples in the Americas had none.
Nearly every disease Europeans carried across the Atlantic in the 1500s triggered this kind of epidemic. The list of Old World diseases absent from the Americas before contact includes smallpox, measles, mumps, chickenpox, influenza, cholera, diphtheria, typhus, malaria, bubonic plague, leprosy, and yellow fever. These didn’t arrive all at once. They rolled through in waves, often decades apart, each one striking communities still recovering from the last.
The Deadliest Outbreaks
Smallpox was the most devastating single disease. It spread faster than European explorers themselves, often reaching communities that had never seen a European. In Mexico, the first major smallpox epidemic hit in 1520 and played a direct role in the fall of the Aztec Empire. But the worst was yet to come.
Between 1545 and 1548, a mysterious illness the Aztecs called cocoliztli swept through Mexico and killed an estimated 5 to 15 million people, wiping out up to 80 percent of the surviving native population. The disease struck again in 1576. Colonial censuses from 157 districts recorded the population dropping from about 2.1 million to just over 1 million in two years, a loss of 51 percent. Researchers still don’t know exactly what cocoliztli was. It caused hemorrhagic fevers, and candidates range from a Salmonella strain to viral hemorrhagic diseases, but no definitive identification has been made.
In North America, the pattern repeated across centuries. The first well-documented widespread epidemic in what would become New Mexico was smallpox in 1636, followed shortly by measles, which killed as many as a quarter of the Pueblo inhabitants. The 1780s smallpox epidemic tore through the Hudson Bay region, with some estimates of mortality exceeding 50 percent, though more recent analysis suggests the figure in that specific region may have been closer to 20 percent.
The 1837 Epidemic and the Mandan
One of the most devastating and best-documented outbreaks hit the northern Great Plains in the summer of 1837, when a steamboat carried smallpox up the Missouri River to trading posts. The Mandan, a farming people who lived in earth-lodge villages near present-day Bismarck, North Dakota, were nearly annihilated. Of approximately 600 Mandan living at the village near Fort Clark, only 14 survived. Across the tribe as a whole, 98 percent died. The neighboring Hidatsa lost a third of their people.
The deaths came so fast and in such numbers that the living could not bury the dead. Bodies were left in lodges or on the edges of the village. The trader Francis Chardon, who kept a daily journal at Fort Clark, eventually stopped recording names or numbers because there were too many. The Mandan’s thirteen clans collapsed to four. Entire ceremonial societies vanished because no one who knew the rituals survived. Sacred bundles, including all but one chief bundle, were lost. The survivors were too few to defend themselves or maintain an independent community, and they merged with Hidatsa and other groups at a new settlement called Like-A-Fishhook Village.
Why Disease Killed More Than Warfare
The sheer scale of disease mortality dwarfed deaths from direct military violence. In many regions, epidemics reduced populations by half or more before significant armed conflict even began. Hernando de Soto’s expedition through the American Southeast in the 1540s encountered thriving chiefdoms; when later Europeans arrived a century later, many of those communities had already collapsed. Disease traveled along trade routes, carried by Indigenous traders who had no idea they were infected.
But disease and colonial violence were deeply intertwined. Displacement from ancestral lands destroyed the food systems that kept people healthy. Communities forced onto unfamiliar territory or confined to small areas lost access to the hunting, fishing, and farming practices that had sustained them for generations. Malnutrition weakened immune responses and made infections more lethal. Warfare scattered communities, broke down social structures that organized care for the sick, and created the crowded, unsanitary conditions where epidemics spread fastest.
Children removed from their families and placed in institutional settings lost access to traditional diets and were exposed to new pathogens in close quarters. The cycle was self-reinforcing: disease weakened communities, making them more vulnerable to military defeat and displacement, which in turn made them more vulnerable to the next epidemic.
Putting the Numbers in Perspective
If the pre-contact population of the Americas was around 50 million and the total decline reached 90 percent, approximately 45 million Indigenous people died in the first 150 years of contact. Disease caused the majority of those deaths. Some scholars who favor higher starting populations put the toll significantly above that. Using the Berkeley school’s estimates of 100 million or more, the death toll from all causes could exceed 90 million.
For what is now the United States specifically, the Indigenous population fell from somewhere between 5 and 15 million to roughly 250,000 by the late 1800s. That represents a decline of 95 to 98 percent over four centuries, with disease responsible for the largest share at every stage. By any estimate, the demographic collapse of Indigenous peoples in the Americas was one of the largest mortality events in human history.