Disease and European Expansion to the Americas 

After Christopher Columbus landed in San Salvador in 1492, Europeans settled and eventually conquered the Americas. How did Europeans overwhelm an Indigenous population that scholars have estimated to be between 60 and 100 million? An essential factor involves the drastic population decrease of Native Americans over the following two centuries. The estimated numbers are staggering, ranging between 70 to 95%!

Scholars cite numerous reasons for this devastation, including superior European technology (e.g., weapons), brutal European tactics, and disease.  Of these factors, diseases carried by Europeans stand as the main culprit of the Native American decimation. Europeans also succumbed to illness but not nearly to the extent of the Indigenous population.

Why did the exchange of germs between Europeans and Native Americans lead to such lopsided results? What gave Europeans a more resilient immune system? To find answers, we need to venture back thousands of years.

Farming and Livestock. Scholars like Alfred Crosby and Jared Diamond argue that Europe’s biological advantage stemmed from the Agricultural Revolution that scholars estimate began c 8000 BC. This gradual transition from hunting and gathering to farming led to significant changes that would facilitate European expansion many centuries later.

Two overarching factors come into play. The first is the domestication of diverse animal species. Alfred Crosby points out that the Old World domesticated a wider range of animals than New World communities. Compare, he writes, “the American assemblage of livestock (dog, lamas, guinea pigs, and some fowl), with that of the Old World: (dogs, cats, cattle, horses, pigs, sheep, goat, reindeer, water buffalo, chickens, geese, ducks, horses and more.” (19)

Ancient Sumerians (c4500-1900BC), residing in the southern part of Mesopotamia, in the flatlands between the Tigris and Euphrates rivers (now south-central Iraq), stood as the eminent domesticators of animals in their time. Animal power allowed people to farm more land than human muscle could muster. Livestock provided a consistent food source – meat and milk – essential during poor cop seasons. Both factors yielded more food, more surplus and facilitated population growth.

More livestock also meant more interspecies exchange of a broader range of microbes. Animals and people died or became quite sick, but sustained contact gradually led to immunity to a comprehensive range of diseases. Smallpox, the flu, influenza, measles, and other ailments evolved from human and livestock interactions. Poxviruses, Crosby points out by way of example, “oscillated back and forth between humans and cattle to produce smallpox and cowpox.” (Crosby, 31) 

Population and “Crowd Disease” Secondly, higher and denser populations exacerbated this interplay of livestock and human. As technologies improved, more people (and animals) could live in smaller areas and encouraged greater microbial spread. The rise of cities further encouraged the spread of germs. More people breathe the same air, spend more time in proximity and are more likely to contact human waste and disease carriers like rodents and insects that thrived in dense human populations.

Of course, these increasingly immunized people from cities and large villages did not stay put. Sumerians and subsequent civilizations traded, travelled, moved, and fought battles and wars, leading to contact with other peoples, including hunter-gathers who lacked sophisticated immune systems. 

Centuries later, when Columbus reached the Americas, Europeans had developed an incredibly resilient immune system that could withstand the likes of smallpox, yellow fever, diphtheria, influenza, chickenpox, and a host of other diseases bred over centuries. Native Americans lacked the immune systems to cope with the microbial onslaught. American populations, new studies show, were higher and denser than previously assumed but still not comparable to European ones. As previously mentioned, a less diverse American livestock inventory meant a narrower field of microbial exchange.  

Disease alone did not decimate America’s Indigenous population, but it seems to be the main culprit. “Far more Americans died,” Jared Diamond writes, “in bed from European germs than on the battlefield from European guns and swords.” (201). Other factors certain exacerbated their impact. Thomas Benjamin points out that diseases “were accompanied and made more deadly by war, exploitation, slavery, and missionaries who brought dispersed people together in a large community.” (321). We must also note that diseases did not uniformly impact the Americas. Denser and higher populations saw higher mortalities. Some Native communities in the northern reaches of Canada did not feel the impact of Old World disease until the 19th and 20th centuries.    

Scholars still debate the topics such as population numbers and whether Europeans, at times, intentionally infected Native people. However, there is a growing consensus that Old World diseases led to the European conquest of the Americas more than any other factor. 


Columbian Exchange. Alfred Crosby coined this term to describe the transcontinental transport and exchange of plants, animals, and diseases. 

Crowd Disease. A disease that can only be exchanged from person to person and therefore thrive in crowded populations.

Influenza. Flu, contagious respiratory disease.

Measles. A viral disease marked by red spots on the skin.

Microbes. A microorganism. Often referring to bacterium causing disease.

Pathogen. An agent that causes disease.

Smallpox. Viral disease became more virulent during the Renaissance (coinciding with Columbus’s voyage to the Americas). Arguably the most devasting disease to Native Americans. 

Selected Bibliography.

Benjamin, Thomas. The Atlantic World: Europeans, Africans, Indians, and Their Shared History, 1400-1900. New York: Cambridge University Press, 2009.

Crawford, Dorothy H. Deadly Companions: How Microbes Shaped our History. Oxford: Oxford University Press, 2017.

Crosby, Alfred W. Ecological Imperialism: The Biological Expansion of Europe, 900-1900. New Edition. New York: Cambridge University Press, 2004.

Diamond, Jared. Guns, Germs, and Steel. The Fates of Human Societies. New York: W.W. Norton and Company, 2017.

Hopkins, Donald R.  The Greatest Killer: Smallpox in History. Chicago: University of Chicago Press, 2002.

Kiple, Kenneth F. ed. The Cambridge World History of Human Disease. Cambridge: Cambridge University Press, 1993.

Martin, Charles. A Short History of Disease: Plagues, Poxes and Civilizations. Harpenden, Herts: Pocket Essentials, 2015. 

Watts, Sheldon. Epidemics and History: Disease, Power, and Imperialism. New Haven: Yale University Press, 1997.