History › Early modern › Columbian Exchange
Epidemics in the Americas
from 1518 (smallpox); populations fell by perhaps 50–90%· Event· Americas & Oceania
DebatedHistorians or scientists disagree about what happened, when, or why.
Diseases brought from Europe and Africa, above all smallpox, killed a large share of the Americas' native peoples from the 1500s on.
See it on the History map →What happened
Isolated for millennia, Native Americans had no immunity to smallpox, measles, influenza or typhus. Smallpox reached Hispaniola in 1518 and Tenochtitlan in 1520, then spread ahead of the Spanish toward the Andes. Waves of disease, worsened by war, forced labor, famine and displacement, recurred for over a century. Estimates of the population before 1492 range from about 8 million to over 100 million; many scholars now suggest roughly 50–60 million, falling by perhaps 50–90%.
Why it matters
Probably the largest population collapse relative to size in human history. It weakened resistance to conquest and led colonists to import enslaved Africans as replacement labor.
How we know
Spanish and Indigenous accounts, colonial tribute and census records, skeletons and mass graves, and ancient DNA, such as a 2018 study linking a 1545 Mexican epidemic to a Salmonella strain.
Still debated
The size of pre-1492 populations, and how much of the collapse was disease versus violence and exploitation, remain hotly debated.
Connections (3)
▸Epidemics in the Americas killed the emperor of Inca EmpireMostly agreed
The connection
An epidemic, most likely smallpox, apparently spread south from Central America ahead of the Spanish. Around 1527 it killed the Inca ruler Huayna Capac and his chosen heir, Ninan Cuyochi. With no clear successor, his sons Huáscar and Atahualpa fought a bitter civil war, which Atahualpa had just won when Pizarro arrived in 1532.
When and how
Ecuador and Peru, about 1524–1528; Huayna Capac died near Quito; his sons Huáscar, based in Cusco, and Atahualpa, based in Quito.
How sure are we?
Spanish chroniclers such as Cieza de León and accounts by Inca descendants describe a deadly epidemic killing him, though all were written decades later.
Debates and open questions
The disease is not certain: smallpox is most likely, but some scholars suggest measles or another illness. His death date is given as 1524 to 1528.
▸Columbian Exchange brought the diseases behind Epidemics in the AmericasWell documented
The connection
Ships and settlers carried smallpox, measles, influenza and typhus from Europe, and malaria and yellow fever from Africa via the slave trade. Many of these diseases arose in Eurasia's crowded farming towns, jumping from livestock. The Americas had few domesticated animals and fewer such diseases, so Native Americans lacked acquired immunity.
When and how
The Caribbean from 1493; smallpox from 1518; Mexico in 1520; the Andes by the late 1520s; North America through the 1600s and later.
How we know
Spanish and Indigenous descriptions of symptoms, timing that follows contact, and ancient DNA identifying European and African pathogens in colonial-era remains.
Debates and open questions
How much genetics versus malnutrition, forced labor and violence raised death rates is debated, as is which disease caused some epidemics.
▸Epidemics in the Americas helped make possible Spanish conquest of the Aztec and Inca empiresMostly agreed
The connection
Smallpox struck Tenochtitlan in 1520, just after the Spanish were driven out, killing perhaps a third or more of the city, including the new emperor Cuitláhuac; estimates vary. In the Andes, disease killed the Inca ruler and set off civil war before Pizarro arrived. Epidemics left defenders sick, leaderless and grieving at decisive moments.
When and how
Tenochtitlan, autumn 1520; the Andes, about 1524–1532. Spanish sources say a sick man with Pánfilo de Narváez's expedition brought smallpox to Mexico.
How sure are we?
Nahua accounts in the Florentine Codex describe the smallpox epidemic; Spanish chroniclers record its timing alongside the conquest.
Debates and open questions
How decisive disease was, compared with Native allies and Spanish weapons, is debated; some historians think it is overemphasized, downplaying conquest's violence.