Disease and the Age of Exploration

Of all the consequences of the Age of Exploration, none was more catastrophic than the spread of Old World diseases to the Americas. When Europeans crossed the Atlantic beginning in 1492, they carried with them a suite of infectious diseases against which the indigenous peoples of the Americas had no inherited immunity. The resulting epidemics killed millions, perhaps tens of millions, depopulated entire regions, toppled powerful empires, and permanently altered the demographic and political map of the New World. The demographic collapse was, in the words of Noble David Cook and W. George Lovell, “the greatest demographic disaster in the history of the human species.”

The history of disease in the Age of Exploration is not, however, a single story. It is a set of distinct epidemiological events that played out differently in different regions and that historians, working with incomplete records, are still struggling to reconstruct. The first major epidemic in the Americas struck Hispaniola in 1518 or 1519. Smallpox reached central Mexico in 1520 with the Narváez expedition, killing the Aztec emperor Cuitláhuac and helping to clear the way for Cortés. Smallpox reached the Andes around 1524 or 1525, killing the Inca emperor Huayna Capac around 1527 and triggering the civil war between Atahualpa and Huáscar that left the empire fatally divided when Pizarro arrived. The 1576 cocoliztli epidemic in central Mexico — widely held, on the basis of recent paleomicrobiological work, to have been a viral hemorrhagic fever rather than smallpox — killed perhaps half of the remaining indigenous population. The 1616–1619 epidemic in New England cleared the way for English colonization of lands recently depopulated of their original inhabitants. Each of these events was a regional catastrophe; together, they constituted a continental one.

The Diseases Themselves

The Old World pathogens that reached the Americas after 1492 included the most lethal infectious diseases of pre-modern Europe and Asia. The most important were smallpox, measles, influenza, typhus, bubonic plague, diphtheria, cholera, scarlet fever, whooping cough, malaria, and yellow fever. Some of these — malaria and yellow fever were originally tropical African diseases that had already spread to the Mediterranean — also moved in the opposite direction, becoming established in the New World with devastating consequences for Europeans as well.

Smallpox

Smallpox, caused by the variola virus, was the most lethal of the imported diseases. It killed roughly 30 percent of those infected and left many survivors blinded or disfigured. The first major epidemic in the Americas struck Hispaniola in late 1518 or early 1519, killing much of the indigenous population of the island. Subsequent epidemics spread to Mexico, Central America, the Caribbean, and South America. The full story is told on the smallpox and indigenous populations page.

Measles

Measles, caused by a paramyxovirus, generally killed a smaller percentage of those infected than smallpox but spread rapidly through populations with no prior exposure. In unexposed populations, measles can kill 20 to 30 percent of those infected, particularly children. In the Americas, measles epidemics often followed smallpox epidemics by a year or two, claiming large numbers of survivors still weakened by the earlier disease.

Influenza

Influenza, in its pandemic forms, was devastating to Native Americans. The 1556 influenza epidemic is reported to have killed large numbers of people in Mesoamerica and the Andes, and earlier waves may have reached the Americas with the earliest European contacts. J.R. McNeill’s Mosquito Empires (2010) places influenza and other epidemic diseases at the center of the demographic story of the Caribbean.

Typhus, Bubonic Plague, and Other Diseases

Typhus, transmitted by body lice, killed large numbers of people in Mexico and the Andes in the 16th century. Bubonic plague, the Black Death of medieval Europe, was a less common visitor to the Americas but did appear in some port cities. Diphtheria, scarlet fever, whooping cough, and other childhood diseases killed many children and contributed to the long-term demographic decline of indigenous populations. Malaria and yellow fever, both mosquito-borne, became major killers in the tropical Americas, claiming European as well as indigenous lives. The 1793 yellow fever epidemic in Philadelphia, which killed roughly 10 percent of the city’s population, was the most spectacular American outbreak.

Why Native Americans Were So Vulnerable

The extreme vulnerability of indigenous Americans to Old World diseases is one of the central questions in the history of the Columbian Exchange. The short answer is that the indigenous populations of the Americas had been isolated from the Old World for roughly 10,000 to 15,000 years and had never been exposed to the disease pool of Eurasia and Africa.

The Role of Domestic Animals

Most of the deadliest Old World diseases are zoonoses — diseases that originated in animals and jumped to humans. Smallpox is thought to have evolved from a related virus in African or Asian rodents (or, in some accounts, cattle); measles from rinderpest or canine distemper; influenza from pigs or ducks; typhus from rodents via body lice. Eurasian populations had lived in close proximity to domesticated animals — cattle, pigs, sheep, goats, horses, chickens — for thousands of years. Repeated epidemics over centuries killed many of those who lacked resistance but left descendants with greater immunity. By 1492, most Europeans had at least partial genetic and immunological defenses against the major epidemic diseases.

Native Americans, by contrast, had domesticated very few large mammals. The principal New World domestic animals were the llama, alpaca, guinea pig, and turkey in the Andes and the dog in Mesoamerica and North America. None of these animals is a major reservoir for diseases that readily infect humans. The result was that when Eurasian pathogens arrived in the Americas, they encountered populations with virtually no prior exposure and almost no inherited immunity.

The Lack of Acquired Immunity

In addition to lacking genetic resistance, indigenous Americans had no acquired immunity. In Old World populations, childhood exposure to milder forms of diseases like measles and chickenpox provided partial protection against future infection. Native Americans of all ages were equally vulnerable, which meant that a single epidemic could kill not only children but the entire adult population in the worst-affected communities.

The Timing and Spread of Epidemics

The first major European contact with the Americas was Columbus’s arrival in the Caribbean in 1492, but the great epidemic waves did not begin until the early 16th century. The pattern of spread is well documented for the most heavily affected regions.

Hispaniola and the Caribbean

The first recorded epidemic among the indigenous peoples of the Caribbean struck Hispaniola in 1518 or 1519, an outbreak of smallpox. It is estimated to have killed a substantial portion of the native Taíno population, which had already been reduced by enslavement, warfare, and other diseases. Subsequent epidemics of measles, influenza, and other diseases continued to depopulate the islands. The Taíno population of Hispaniola, estimated at several hundred thousand in 1492, was reduced to a few thousand by the mid-16th century and effectively disappeared as a distinct people within a few generations.

Mexico and Central America

The first smallpox epidemic in mainland America reached central Mexico in 1520, carried by an enslaved African in the Pánfilo de Narváez expedition sent to arrest Hernán Cortés. The epidemic swept through the Aztec capital of Tenochtitlán and surrounding regions, killing perhaps 25 to 40 percent of the population of central Mexico, including the emperor Cuitláhuac. It returned repeatedly through the 16th century, and was followed by epidemics of measles, typhus, and other diseases. The 1576 cocoliztli epidemic, now thought to have been caused by a viral hemorrhagic fever rather than smallpox, killed perhaps half of the remaining indigenous population of central Mexico. The result was a demographic collapse of enormous scale: the population of central Mexico fell from an estimated 20 to 30 million in 1519 to perhaps 2 to 3 million by 1600.

The Andes

Smallpox reached the Inca Empire before the Spanish. The disease was carried overland from Central America or possibly by shipwrecked Spaniards sailing along the Pacific coast, and it struck the Inca heartland around 1524 or 1525. The emperor Huayna Capac is generally believed to have died a year or two after the epidemic took hold, around 1527, along with much of his court. The epidemic triggered a civil war of succession between Huayna Capac’s sons Atahualpa and Huáscar that left the empire fatally divided when Francisco Pizarro arrived in 1532. Subsequent epidemics further reduced Andean populations, with the highland regions of Peru and Bolivia losing an estimated 80 percent or more of their pre-Columbian inhabitants.

North America

Epidemic disease reached North America more gradually. The first documented smallpox epidemic in the present-day United States struck the Timucua people of Florida in the 1520s. Major epidemics swept through the Pueblo Southwest, the Mississippi valley, and the eastern woodlands in the 16th, 17th, and 18th centuries. The 1616–1619 epidemic, possibly leptospirosis or some combination of diseases introduced by European fishermen and explorers, devastated the New England coast, killing perhaps 90 percent of the native population in some areas and clearing the region for English colonization. The 1837–1838 smallpox epidemic on the Great Plains, the subject of recent work by Jeffrey Ostler and others, struck the Mandan, Hidatsa, and Arikara with devastating mortality, contributing to the weakening of indigenous resistance to American expansion.

Disease as an Ally of Conquest

The role of disease in the Spanish conquests of the Aztec and Inca empires is among the most studied topics in the history of exploration. The conventional view — that tiny bands of Spanish conquistadors defeated vast indigenous empires through superior military technology, horses, and cunning — has been complicated by a growing appreciation of the demographic catastrophe that preceded direct contact.

When Cortés landed on the coast of Mexico in 1519, the Aztec Empire was at the height of its power. Within two years, it had fallen to a few hundred Spanish soldiers and many indigenous allies. Spanish military advantages — steel weapons, firearms, horses, armor, and tactical experience — were real but not in themselves sufficient to overcome the millions of warriors available to the Aztec tlatoani Moctezuma. What made the conquest possible was the smallpox epidemic that arrived in 1520, killing Moctezuma himself and much of the Aztec ruling class, and a succession of follow-up epidemics in subsequent years that continued to weaken the empire.

Cortés and his allies also exploited the political fractures within the Aztec Empire. Subject peoples such as the Tlaxcalans joined the Spanish against their Aztec overlords, and the conquest was as much a civil war within Mesoamerica as a foreign invasion. The same pattern unfolded in the Andes, where the Cañari, the Chachapoya, and other conquered groups joined Pizarro against the Incas. The conquests were, in Matthew Restall’s phrase, “messy, contingent, and indigenous-supported Spanish enterprise[s].”

Estimating the Demographic Catastrophe

The scale of indigenous depopulation in the Americas is among the most debated topics in the field. The 80 to 95 percent figure for the worst-affected regions is a serious scholarly range, not a precise measurement. Estimates of the pre-Columbian population range from about 60 million (a traditional figure associated with Kroeber, Rivet, and others) to over 100 million (a higher figure associated with the revisionist demographer Henry F. Dobyns and adopted by Charles Mann in 1493). The exact numbers remain uncertain, but the qualitative conclusion is undisputed: between 1492 and the early 1600s, the indigenous population of the Americas was reduced by a staggering proportion, primarily through disease rather than direct violence.

In the Caribbean, the population collapse was nearly total: the native peoples of the major islands were effectively extinct within a few generations. In central Mexico, the population may have fallen from 20 to 30 million to 2 to 3 million. In the Andes, similar declines occurred. In some areas, the population did not begin to recover until the 18th or 19th century, and in some regions of the Amazon and interior South America, indigenous populations continued to decline into the 20th century.

Sources of Population Loss

The bulk of the loss was due to disease rather than direct violence, although war, enslavement, and forced labor contributed. The pattern of depopulation, in which communities were struck by successive waves of different diseases over many decades, suggests that the cumulative impact of repeated epidemics was more important than any single outbreak. The 1576 cocoliztli in central Mexico, the 1616–1619 New England epidemic, and the 18th- and 19th-century smallpox epidemics on the Plains were all distinct events, each devastating in its own right, that together produced the cumulative demographic catastrophe.

The Columbian Exchange in Reverse

The disease exchange was not entirely one-way. While Old World pathogens devastated the New World, New World diseases had less impact on Europe and Africa — in part because they were diseases like syphilis that had been previously rare or unknown in the Old World. Syphilis, in particular, appeared in Europe in the 1490s and spread rapidly across the continent, although its severity and origins have been debated for centuries. The extent to which the disease was actually new to Europe remains contested; some scholars, following the recent genetic work of Harper and others, have argued that treponemal disease was present in Europe in mild form before 1492 and that the 1490s epidemic was the result of a mutation into a more virulent strain rather than a true new-world import. The history of disease and exploration is in this respect a two-sided story.

Long-Term Consequences

The demographic catastrophe unleashed by disease reshaped the Americas in ways that are still visible. The political and economic vacuum created by the collapse of indigenous populations allowed European empires to establish themselves across the hemisphere. The labor shortage caused by indigenous depopulation was a major driver of the Atlantic slave trade, which in turn reshaped Africa and the Americas. The cultural trauma of epidemic disease left deep marks on surviving indigenous communities, many of which incorporated the experience of plague into their oral traditions, religious practices, and historical narratives.

The history of disease in the Age of Exploration is, in retrospect, a story of unintended consequences. European explorers, soldiers, and missionaries did not set out to depopulate the Americas through disease. But the biological separation of the hemispheres meant that contact with the Old World was, for indigenous Americans, a demographic catastrophe without parallel in human history.