Europeans introduced smallpox, measles, influenza, mumps, tuberculosis, diphtheria, and several other infectious diseases to the Americas beginning in 1492. These diseases killed tens of millions of Indigenous people over the following centuries, wiping out entire communities and reshaping the political landscape of two continents. The scale of death was unlike anything in recorded history, with some populations losing 90% or more of their people within a few generations of contact.
The Major Diseases
The deadliest diseases Europeans carried across the Atlantic are sometimes called “civilization pathogens” because they evolved alongside densely settled agricultural societies in Eurasia. The most devastating include:
- Smallpox: The single most destructive disease introduced to the Americas. It spread faster than Europeans themselves could travel, sometimes reaching communities years before they ever encountered a European person. Among the Haida people of Alaska, one outbreak killed 70% of the population.
- Measles: Particularly lethal in populations with no prior exposure. Among Northern Plains Indigenous groups, measles epidemics killed up to 50% of those infected.
- Influenza: Respiratory illness swept through communities with extreme severity. On St. Lawrence Island in Alaska, up to 22% of Native Alaskans died in a single influenza outbreak, with regional mortality reaching 75%.
- Tuberculosis: A slow-moving lung disease that became endemic in many Indigenous communities and continued killing for centuries.
- Mumps and diphtheria: Less well-documented individually but part of the relentless cycle of new infections that struck one after another.
All of these pathogens shared a common trait: they had evolved from diseases carried by domesticated animals in western Eurasia. Thousands of years of close contact between humans, cattle, pigs, and other livestock in Europe, Asia, and Africa created a breeding ground for viruses and bacteria that could jump between species. Indigenous peoples of the Americas had domesticated far fewer animal species, so they had never been exposed to this family of diseases.
Diseases That Came From Africa
Not every disease arrived directly from Europe. Yellow fever almost certainly originated in Africa and crossed the Atlantic during the slave trade in the 15th and 16th centuries. The virus may have first reached the Caribbean as early as Columbus’s voyages between 1492 and 1495, possibly carried by crews who had resupplied in the Canary Islands off the African coast. It traveled in the eggs of a specific mosquito species that can survive months without water, allowing the insects to make the ocean crossing aboard ships.
Once yellow fever reached the tropical Americas, it found ideal ecological conditions and established a permanent cycle in local mosquito and primate populations. It became a recurring killer in port cities and lowland areas throughout the Caribbean, Central America, and South America for the next four centuries.
Why Indigenous People Were So Vulnerable
The catastrophic death tolls were driven by what epidemiologists call “virgin soil” epidemics, outbreaks that strike populations with zero prior exposure to a disease. Every single person who encountered the pathogen was susceptible. In a European city, smallpox might kill children who hadn’t yet been exposed, but most adults had survived it in childhood and carried immunity. In the Americas, the virus hit adults, children, and elders simultaneously.
Making things worse, surviving one disease offered no protection against the next. A community that endured a smallpox epidemic and developed immunity to it remained completely defenseless against measles, influenza, or any of the other pathogens that followed. These diseases often arrived in waves, hammering communities that were still recovering from the previous outbreak. People who were weakened, grieving, or unable to plant and harvest food became even more vulnerable to the next infection.
The Scale of Population Loss
The numbers are staggering even by modern standards. In the Jemez Province of what is now New Mexico, the population fell from roughly 6,500 people to fewer than 850 between 1620 and 1680, a loss of 87%. Disease was the primary driver, compounded by violence and famine. This pattern repeated across the hemisphere. Communities didn’t just lose people. They lost farmers, healers, leaders, parents, and the elders who carried oral histories and cultural knowledge.
Across large regions of the Americas, entire villages were abandoned. The combination of disease, warfare, and starvation created a cascading collapse. When enough adults fell sick simultaneously, no one could care for the ill, gather food, or maintain the social structures that held communities together. Mortality rates in these conditions climbed far higher than the disease alone would have caused.
Disease and the Fall of Empires
The timing of epidemics played a direct role in the Spanish conquests of both the Aztec and Inca empires. Disease didn’t just thin populations. It killed leaders and threw political systems into chaos at the worst possible moment.
In the Aztec capital of Tenochtitlan, smallpox struck during the Spanish siege. When the Aztec ruler Moctezuma died, his nephew Cuitláhuac was elected to replace him, only to die of smallpox himself shortly after. The rapid loss of leadership destabilized Aztec resistance at a critical point. Spanish forces numbered only in the hundreds, and historians have long noted that disease functioned as an invisible ally worth many hundreds of additional soldiers to Hernán Cortés.
In South America, the pattern was even more striking. A devastating respiratory epidemic swept through the Inca Empire in the 1520s, possibly before any Spaniard had set foot in the region. Indigenous people near Lima later told Spanish chroniclers that the conquest would have been impossible if disease had not “consumed the greater part of them” in the years before Francisco Pizarro arrived. The epidemic killed the Inca emperor Huayna Capac and triggered a civil war between his sons, fracturing the empire just as the Spanish appeared.
The Mystery of Cocoliztli
Some of the worst epidemics in the Americas were caused by pathogens that remained unidentified for centuries. Between 1545 and 1550, a devastating plague swept through Mexico that Indigenous people called “cocoliztli.” It killed millions, but its cause was debated for over a hundred years.
In 2018, researchers used ancient DNA analysis on remains buried in an epidemic cemetery in Oaxaca, southern Mexico, and identified a strong candidate: a strain of Salmonella that causes enteric fever, a severe intestinal illness. This particular strain was likely introduced from Europe, and the combination of a new pathogen, malnutrition, and social disruption from colonization created conditions for a catastrophic outbreak. The discovery highlighted how many of the deadliest post-contact epidemics may have been caused by diseases that historians hadn’t previously considered.
The Columbian Exchange Went Both Ways
One disease may have traveled in the opposite direction. Syphilis appeared in Europe in the 1490s, and the timing led many historians to argue it was brought back from the Americas by Columbus’s crew. This “Columbian hypothesis” is supported by archaeological evidence showing a long history of related infections in the Americas, with bone lesions characteristic of chronic syphilis found in pre-contact Indigenous remains.
The debate isn’t fully settled. Some researchers argue that syphilis or a closely related disease existed in Europe before 1492, pointing to skeletal evidence from European archaeological sites. Others propose that a related bacterial infection was already present on both continents but evolved into its sexually transmitted form under different environmental and social pressures. Ancient DNA research has confirmed that the family of bacteria responsible for syphilis has a deep history in the Americas, but pinning down exactly when and how it crossed the Atlantic remains an active area of investigation. If the Columbian hypothesis holds, syphilis would be the rare case of a disease moving from the Americas to Europe rather than the reverse.