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Biological Impacts of European Expansion in the Americas

Biological Impacts of European Expansion in the Americas

The arrival of Europeans in the Americas in 1492 precipitated a demographic catastrophe. Although some controversy exists over the size of the pre-Columbian population, it is not unreasonable to suggest that it may have fallen from about 50 to 60 million in 1492 to 6.5 million in 1650, a decline of about 90 percent.

Many factors contributed to this decline, but most researchers agree that a major cause was the introduction of Old World diseases. Because of the isolation of the American continent from the rest of the world, Native Americans had not been exposed to diseases that ravaged the Old World and therefore had not acquired any immunity to them. The most notable killers were smallpox, measles, and influenza, which are spread by face-to-face contact. Smallpox is associated with mortality rates of between 30 and 50 percent. The impact of recent measles epidemics among nonimmune populations in Amazonia and isolated regions, such as in Polynesia and Iceland, indicates that measles might result in equally high levels of mortality. In the fifteenth century, most Europeans would have possessed some immunity to smallpox and measles, having been exposed to them in childhood. Other devastating diseases were plague, typhus, malaria, and yellow fever. These diseases are spread by insects, such as lice, fleas, or mosquitoes, and their incidence is strongly influenced by climatic conditions.


The Caribbean was the first region to experience the devastating impact of Old World diseases. The population of the island of Hispaniola, which today is shared by the Dominican Republic and Haiti, declined from at least one million in 1492 to about eighteen thousand in 1518; by the mid-sixteenth century, only a few hundred were left. Prior to the 1980s scholars thought that the smallpox epidemic of 1518 was the first to hit the region, but even accepting that the ill treatment and sometimes the enslavement of indigenous peoples contributed significantly to the decline, the scale of the demographic catastrophe was difficult to explain. However, it is now thought that the island was struck by influenza contracted from pigs suffering from swine fever that were carried by the second expedition of Christopher Columbus (1451–1506) in 1493. A number of other unspecified diseases might also have taken their toll of Caribbean populations before 1518.

The first known epidemic to afflict the American mainland was probably associated with the arrival of smallpox in the Caribbean in 1518, but it is possible that earlier expeditions, such as that of Francisco Hernández de Córdoba (?–1517) in 1517, introduced smallpox at an earlier date to the Maya of Yucatán. What is known is that in 1520 smallpox was carried from Cuba to the Gulf Coast by a sick African slave on the expedition of Pánfilo de Narváez (ca. 1480–1528). It spread rapidly through the native population, devastating the Aztec capital, Tenochtitlán. This weakened its inhabitants physically and psychologically, and shifted the military advantage the Aztecs had possessed to Hernán Cortés (1484–1547) and his troops, who took control of the city in 1521.

One contemporary observer, Toribio de Motolinia (d. 1569), claimed that in most provinces of Mexico, more than half the population died. Further south, smallpox spread through Guatemala and then through native population chains, often arriving ahead of the invaders. In the mid-1520s it struck the Andes in western South America, where it not only caused high morality but also resulted in the death of the Inca emperor, Huayna Capac sometime between 1524 and 1527. His death precipitated a dynastic war between his two sons, Huascar (d. 1532) and Atahualpa (d. 1533), raising mortality to even higher levels and weakening Inca resistance to the Spanish invaders. The Incas claimed that if it had not been for the epidemic, they would not have been conquered so easily.


The initial impact of epidemics was devastating, but native societies were hit by Old World diseases not once but several times during the sixteenth century. For example, in the Andes the smallpox epidemic of the mid-1520s was followed by an outbreak of measles from 1531 to 1533 and by plague or typhus in the 1545 to 1546 period; from 1557 to 1562 the region was struck by measles, along with influenza and smallpox, and finally from 1585 to 1591 by smallpox, measles, and typhus. Since each epidemic carried off a significant proportion of the survivors, a population could be easily hammered down to a fraction of its preconquest size.

This chronology was repeated in many parts of the Americas, but even those societies that did not come into direct contact with conquistadors, explorers, or missionaries may not have been spared the ravages of disease. Infections might spread equally easily through native contacts and systems of exchange. As a result, there is considerable debate over the extent of depopulation that might have occurred in relatively remote areas, such as Amazonia and much of North America, prior to the arrival of Europeans.

During the early colonial period, population losses were higher in the lowlands than the highlands. Many people have explained the greater depopulation in the lowlands by reference to the introduction of malaria and yellow fever, which only thrive in warm climates where the average temperature exceeds 20 degrees centigrade (68 degrees Fahrenheit). Both diseases are spread by mosquitoes, but in the case of yellow fever the particular mosquito that acts as a carrier for the parasite would also need to have been introduced. It is generally thought that it arrived in the New World as the African slave trade expanded and that the first definitely identifiable epidemic of yellow fever occurred in the Caribbean in 1647 to 1648.

Estimated Native American populations, 1492–1996
Source: The estimates for 1570 and 1650–1700 are from Angel Rosenblat, La población indígena y el mestizaje en América, 1492–1950, 2 tomos (Buenos Aires: Editorial Nova, 1954), I, p. 59 and Peter C. Mancall, "Native Americans and Europeans in English America, 1500–1700," Table 15.1, "Indian and colonist demography, 1500–1700," in Nicolas Canney, Ed. The Oxford History of the British Empire. Volume I: The Origins of Empire: British Overseas Enterprise to the Close of the Seventeenth Century (Oxford: Oxford University Press, 1998), p. 331. The estimate for 1820 is from Angus Maddison, The World Economy: Historical Statistics (Paris: OECD, Development Centre, 2003), Table 4.2, "Ethnic Composition of the Americas in 1820," p. 115. The estimate for 1996 is Emma Pearce, "Appendix 1: Indigenous Population Figures." In Phillip Wearne, Return of the Indian: Conquest and Revival in the Americas (Philadelphia: Temple University Press, 1996), pp. 204-215.

Yellow fever came to be associated with port cities, even extending to temperate zones in the summer, where it occasionally afflicted such cities as Boston, Philadelphia, and New York. Malaria may have spread more rapidly because many Europeans already suffered from a mild form of the disease (Plasmodium vivax) and the mosquitoes required for its spread were already present in the New World. However, apparently healthy slaves from Africa probably introduced the more deadly form, Plasmodium falciparum.


The initial impact of Old World diseases was disastrous, but over time native populations began to develop resistance to some infections. Some individuals have innate immunity to infections because of their genetic, biochemical, or physiological makeup, but most acquire it through constant exposure to infections. A community acquires immunity as those people who are resistant survive and reproduce, while those who are not resistant die in childhood.

Historical experience suggests that at least a century of constant exposure is required for an infection to become an endemic and a more benign disease of child-hood. There is evidence that smallpox was becoming a childhood disease in parts of the Andes in the early seventeenth century. However, this type of immunity can only be acquired where communities are sufficiently large to sustain infections indefinitely, so that the population is constantly exposed to them. It has been suggested that a population of between one hundred thousand and two hundred thousand is needed to sustain smallpox.

In areas of low population density, diseases tend to "fade out" because they can find no new susceptible people to infect. In such circumstances, populations are not constantly exposed to diseases and do not therefore acquire immunity to them, with the result that when diseases are reintroduced from the outside these communities continue to experience high mortality.

Following the initial impact of Old World diseases in the sixteenth century, therefore, populations in densely settled areas experienced a degree of recovery, though its timing varied. The native population began to increase in Mexico in the 1620s and 1630s, whereas this did not occur in Central America until the end of the seventeenth century, and in Peru not until the mid-eighteenth century.

Despite this gradual recovery, highland populations were still afflicted by occasional outbreaks and in the eighteenth century there appears to have been a more general resurgence of epidemics that inflicted heavy losses. In Mexico, for example, severe smallpox epidemics occurred in 1711, 1734, 1748, 1761 to 1762, 1779 to 1780, and 1797. Meanwhile, the sparse native populations in the lowlands did not participate in the recovery experienced in the highlands. Here Old World diseases continued to take an elevated toll, so that populations in these regions continued to decline throughout the colonial period.


There is no doubt that the introduction of Old World diseases had a devastating impact on Native American peoples. However, this does not mean that in pre-Columbian times they had lived in a disease-free environment. They suffered from intestinal and respiratory infections, such as diarrhea, dysentery, and tuberculosis, as well as leishmaniasis, Chagas' disease, bartonellosis, and nonvenereal syphilis. What were absent in pre-Columbian times were acute infections that are spread by face-to-face contact and are associated with high mortality.

Sufficiently large populations existed in pre-Columbian times to sustain such diseases if they had existed. However, most human diseases originate in animals and jump the species barrier to become human diseases. It is suggested that the relative absence of domesticated animals in pre-Columbian times and the lack of close contact between humans and the few species that existed would have discouraged their emergence. It seems likely, however, that a form of typhus was present in pre-Columbian times, being carried by lice found on guinea pigs, which were kept in many Andean households.

see also European Explorations in North America; European Explorations in South America; European Migrations to American Colonies, 1492–1820.


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Cook, Noble David. Born to Die: Disease and New World Conquests, 1492–1650. Cambridge, U.K.: Cambridge University Press, 1998.

Cook, Noble David, and W. George Lovell. "Secret Judgments of God": Old World Disease in Colonial Spanish America. Norman: University of Oklahoma Press, 1991.

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