Why did epidemic disease spread only from Europe to America and not both ways in the Columbian Exchange?

Question

In learning about the history of the Americas you often hear about how pre-columbian civilizations operating at a highly advanced level were devastated by epidemic diseases like smallpox, to the point where many of them were practically wiped out and much of the evidence of their habitation reclaimed by nature by the time Europeans were making sustained contact.

I've heard about this in regards to Mississippian civilizations and the inhabitants of the Amazon, among others, and it is cited as the reason why many colonists had the impression of the land as "virgin forest", when it had been previously inhabited by sophisticated civilizations.

I've always wondered why this exchange of disease ended up being so one sided. It seems like in a lot of these cases not much contact was needed for a disease to become rampant (spread through scouting missions or trade networks, hence die off before major European contact), so it seems equally plausible that a European could have brought something back on a ship to which old world populations would have no immunity. Not only that, but population density in parts of the Americas (such as Tenochtitlan) dwarfed that of Western Europe, which seems to suggest it would be an equally probably environment for epidemic disease to originate.

Are there any scientific or historical factors that account for this? Why was the exchange of disease so much more devastating to the American populations?

Answer 1

It is not true that the exchange was one-sided - e.g., a major hypothesis for the origin of Syphilis is that it was brought from Americas:

The history of syphilis has been well studied, but the exact origin of the disease remains unknown. There are two primary hypotheses: one proposes that syphilis was carried to Europe from the Americas by the crew(s) of Christopher Columbus as a byproduct of the Columbian exchange, while the other proposes that syphilis previously existed in Europe but went unrecognized. There has been a recent skeletal discovery in the Yucatan Peninsula dating over 9,900 years ago of a 30 year old woman who had Treponema peritonitis, a disease related to syphilis. "There is also evidence for a possible trepanomal bacterial disease that caused severe alteration of the posterior parietal and occipital bones of the cranium."

Another obvious factor is that, if any Europeans arriving to Americans contracted deadly diseases, they would likely not make it back home. Moreover, Europeans had extensive experience of dealing with infectious diseases - even if they did not understand their nature - dating at back at least to the period of the black death. Notably, quarantining the ship crews for extended periods of time, if anything aroused a suspicion:

In 1448 the Venetian Senate prolonged the waiting period to 40 days, thus giving birth to the term "quarantine". The forty-day quarantine proved to be an effective formula for handling outbreaks of the plague. Dubrovnik was the first city in Europe to set up quarantine sites such as the Lazzarettos of Dubrovnik where arriving ship personnel were held for up to 40 days. According to current estimates, the bubonic plague had a 37-day period from infection to death; therefore, the European quarantines would have been highly successful in determining the health of crews from potential trading and supply ships.

Other diseases lent themselves to the practice of quarantine before and after the devastation of the plague. Those affected by leprosy were historically isolated long-term from society, and attempts were made to check the spread of syphilis in northern Europe after 1492, the advent of yellow fever in Spain at the beginning of the 19th century, and the arrival of Asiatic cholera in 1831.

The fact that most Europeans arriving to Americas were sailors, who had traveled around, were exposed to various infections, and therefore had somewhat higher level of immunity.

Answer 2

@Roger V.'s answer is a good one and I've upvoted it. There are some other additional plausible reasons worth adding:

First (and probably most importantly), mankind evolved in the Old World and this gave diseases many more opportunities to jump to humans. From what we can tell, human diseases mostly are zoonotic in origin. Covid-19 and SARS almost certainly came from animal diseases and jumped to humans in the last fifty years. (This is independent of whether Covid-19 was a lab escape, since if it was, it was from a lab studying animal diseases which might jump to humans!) Green Monkey disease, Marburg virus, and Ebola all appear to have jumped from non-human primates fairly recently. AIDS itself probably has an non-human primate origin. We regularly get influenza strains from birds, but where it evolved first is, as far as I know, unknown -- birds may have caught it from us, originally! Mad Cow disease made the jump to humans less than fifty years ago.

Bottom line: Humans in the Old World lived for longer (it's no accident that Africa is a great source of new diseases) and with more distinct pools of wild animals (giving more opportunities for diseases to evolve) than the people of the New World.

Secondly, the Old World seems to have had greater movement of populations, providing the dubious benefit of regular waves of epidemic disease spreading around. Even as a percentage of the population, many more people died in the Old World than in the New: Bubonic Plague killed half of Europe. The Roman Empire during the reign of Marcus Aurelius suffered from a plague -- unknown which -- which was nearly as deadly. Seventy-five years later the Plague of Cyprian was nearly as deadly. It has been plausibly argued that both came back to Europe with returning armies. And there were many more.

The people of the Old World's "advantage" was that the entire population had been killed off several times over, but not all at once. People in the New World probably got more than one disease at the same time and suffered the same losses all together.

Finally, it's important to remember that people and diseases will in time come to an accommodation. The course of Covid-19 is a good example: When Covid first hit, its effect was pretty serious, but even then it affected immunologically naive old people far more seriously than the immunologically naive young. Today, there are no immunologically naive people of any age left: There are very, very few people who have not either been immunized or gotten Covid or both, and the effect of the disease is greatly muted.

But besides that, Covid itself has mutated to become more contagious. It's done so by preferentially lodging in the upper respiratory system which makes it easier from virus particles to become airborne and go off to conquer new worlds. But the deadliest effect of Covid was infections deep in the lungs, so this evolution -- entirely to Covid's reproductive advantage! -- has muted the danger it poses to humans.

The SARS outbreak in 2003 only infected a few thousand people, but there is some evidence that those people had some natural immunity to Covid (a close relative of SARS) when it came around. And Covid has probably armed all of us with at least some immunity to the next SARS-family virus to make the jump to people. Living in a bigger stew of diseases, Old World people carried both diseases and acquired immunity to those diseases that people in the New World lacked.

This is a grossly over-simplified summary of a large topic, but I think it covers the main points adequately. Two very readable books that go into more detail is the classic Plagues and People by William McNeil and the more recent Guns, Germs and Steel by Jared Diamond.

Answer 3

The Columbian Exchange, which occurred after the arrival of Christopher Columbus in the Americas in 1492, facilitated the exchange of goods, ideas, and microorganisms between the Old World (Europe, Asia, and Africa) and the New World (the Americas). While various elements were exchanged in both directions, including crops, animals, and diseases, the impact of epidemic diseases was more pronounced in the Americas than in Europe. Several factors contributed to this pattern:

Population Density and Domesticated Animals:

Europe had a longer history of dense human populations living in close proximity to domesticated animals. This close association allowed for the transmission and adaptation of diseases between humans and animals. The exchange of diseases often occurred during the process of domestication. Epidemic Diseases in Europe:

By the time of Columbus's voyages, Europe had experienced and, to some extent, developed immunities to various epidemic diseases like smallpox, measles, and influenza. These diseases had been circulating in European populations for centuries. This long-term exposure led to some level of acquired immunity. Lack of Domesticated Animals in the Americas:

Before European contact, the Americas had a smaller population and fewer domesticated animals compared to Europe. The lack of large, domesticated animals like horses, cows, and pigs in the Americas limited the opportunities for diseases to jump from animals to humans (zoonotic transmission). Isolation of the Americas:

The Americas were relatively isolated from the rest of the world before Columbus's arrival. This isolation meant that the indigenous populations had not been exposed to many of the infectious diseases that had long been present in Europe, Africa, and Asia. As a result, the indigenous people lacked immunity to these "Old World" diseases. Devastating Impact of European Diseases:

When Europeans arrived in the Americas, they unintentionally introduced diseases like smallpox, measles, and influenza to which the native populations had no immunity. This lack of immunity led to devastating epidemics that wiped out large percentages of indigenous populations. In summary, the one-sided impact of epidemic diseases from Europe to the Americas in the Columbian Exchange can be attributed to differences in population density, the history of disease exposure in Europe, the lack of domesticated animals in the Americas, and the isolation of the continents. This asymmetry in disease transmission had profound and tragic consequences for the indigenous peoples of the Americas.