What Diseases Were Brought to the Americas by Europeans?

When Europeans began to visit and colonize the Americas after 1492, they brought with them a suite of infectious diseases that had been endemic in the Old World for centuries but were unknown in the New World. Because the indigenous peoples of the Americas had been isolated from the Old World disease pool for roughly 10,000 to 15,000 years, they had no inherited immunity to these pathogens. The result was a series of devastating epidemics that killed perhaps 80 to 95 percent of the pre-Columbian population in the worst-affected regions — the greatest demographic catastrophe in human history. This page catalogs the major diseases that traveled from Europe, Africa, and Asia to the Americas during the Age of Exploration and explains their consequences for the Indigenous Peoples of the Americas.

The full history of disease and exploration is told on the disease and exploration page; the impact of the most important disease is examined on the smallpox and indigenous populations page.

The Major Diseases

The Old World diseases that reached the Americas after 1492 included the most lethal infectious diseases of pre-modern Europe, Africa, and Asia. The following list covers the most important, with brief explanations of their origins, transmission, and impact.

Smallpox

Smallpox, caused by the variola virus, was the most lethal of the imported diseases. It is thought to have evolved from a related virus in African or Asian rodents thousands of years ago and to have become a human disease sometime before 1000 BCE. By the time of Columbus, smallpox had killed hundreds of millions of people in Eurasia and Africa, and surviving populations had developed partial genetic resistance.

Smallpox killed roughly 30 percent of those it infected and left many survivors blinded or disfigured. The first major epidemic in the Americas struck Hispaniola in 1518 or 1519 and rapidly spread to the Caribbean, Mexico, and Central America. The 1520 epidemic in central Mexico killed perhaps 25 to 40 percent of the population, including the Aztec emperor Cuitláhuac. Subsequent epidemics in the 1530s, 1540s, 1550s, and 1560s repeatedly swept through Mesoamerica and the Andes. The full story of smallpox in the Americas is told on the smallpox and indigenous populations page.

Measles

Measles, caused by a paramyxovirus, is closely related to the rinderpest virus of cattle and is thought to have evolved as a human disease after cattle were domesticated. It is highly contagious and spread through respiratory droplets. 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, caused by various strains of the influenza virus (originally an avian disease), has been responsible for some of the deadliest pandemics in human history. In the 16th century, influenza epidemics struck the Americas repeatedly, often in concert with smallpox and measles. The 1556 influenza epidemic, in particular, was devastating in parts of Mesoamerica and South America.

Typhus and Bubonic Plague

Typhus, caused by the bacterium Rickettsia prowazekii and transmitted by body lice, was a frequent companion of famines, wars, and disasters in pre-modern Europe. In the Americas, typhus epidemics occurred in the 16th and 17th centuries, contributing to the depopulation of indigenous communities. Bubonic plague, the disease responsible for the Black Death of medieval Europe, was a less frequent visitor to the Americas but did appear in some port cities.

Childhood Diseases

A number of other Old World diseases, including diphtheria, scarlet fever, whooping cough, mumps, rubella, and chickenpox, also reached the Americas after 1492. These diseases are less spectacular than smallpox or measles, but they had a cumulative effect on indigenous populations, killing many children and reducing fertility among survivors. The cumulative impact of repeated epidemics of childhood diseases drove the long-term depopulation of indigenous America.

Malaria and Yellow Fever

Malaria, caused by Plasmodium parasites transmitted by mosquitoes, was originally an African disease that had spread to the Mediterranean region in antiquity. In the Americas, malaria became established in the tropical lowlands, where it killed both indigenous and European populations. It was a major obstacle to European colonization of the Caribbean and Central America for much of the 16th and 17th centuries. The African genetic adaptations to malaria — particularly the sickle-cell trait — became more prevalent in the African diaspora as a result.

Yellow fever, caused by a virus transmitted by the Aedes aegypti mosquito, also originated in Africa. It became endemic in the Caribbean and tropical mainland Americas, where it killed large numbers of Europeans. Yellow fever was a particular problem for the slave trade, as European and African mortality from the disease was sometimes high. The 1793 yellow fever epidemic in Philadelphia was the most spectacular American outbreak.

Sexually Transmitted Diseases

Syphilis is the most controversial entry on the list. The disease, caused by the bacterium Treponema pallidum, appeared in Europe shortly after Columbus’s return from the Americas in 1493, leading to a long-standing debate about whether it was imported from the New World. Modern genetic evidence supports the hypothesis that syphilis, or a closely related strain of the disease, was indeed present in the Americas before 1492 and was carried to Europe by Columbus’s crew. Syphilis spread rapidly through Europe in the late 15th and 16th centuries, causing severe epidemics. The extent to which the disease was actually new to Europe remains contested; some scholars, following the recent genetic work of Kristin 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.

Why These Diseases Were So Deadly in the Americas

The extreme virulence of Old World diseases in the Americas was a direct consequence of the long biological separation of the Eastern and Western Hemispheres. The major epidemic diseases of Eurasia — smallpox, measles, influenza, typhus, plague — had all evolved in close proximity to Old World populations for thousands of years. Repeated epidemics had eliminated those most susceptible and left survivors with partial genetic or acquired immunity. By 1492, the typical European had survived childhood exposure to many infectious diseases and had developed some level of protection.

Indigenous Americans had no such history. Most of the major epidemic diseases are zoonoses that originated in Old World domesticated animals — cattle, pigs, sheep, goats, horses, and chickens. The New World had very few large domestic animals, so the indigenous peoples of the Americas had no evolutionary exposure to the pathogens that had shaped Old World disease history. When these pathogens arrived, they encountered populations with essentially no immunity, and the result was catastrophic.

The Long-Term Impact

The impact of imported diseases on the indigenous peoples of the Americas is difficult to overstate. Whole cities, regions, and civilizations were depopulated. The Aztec and Inca empires were fatally weakened by epidemics even before the Spanish arrived. The political vacuum created by demographic collapse allowed European empires to establish themselves across the hemisphere, and the labor shortage caused by indigenous depopulation drove the expansion of the Atlantic slave trade.

The history of disease in the Age of Exploration is one of the central topics in the broader Columbian Exchange. The biological, demographic, and cultural consequences of the disease exchange continue to shape the world today.