Traveling Humans, Traveling Disease

Patrick Christmas

Human culture took an enormous leap forward with the gradual transition to a sedentary lifestyle with agriculture and the domestication of animals to provide subsistence. The switch affected us as organisms in countless ways, many stemming from the fact that we had evolved over thousands of years with a hunter-gatherer mode of living. Not surprisingly, this also had just as many consequences with respect to the environment we live in; obvious ones would include massive land use for growing crops and the concentration of our populations in the form of towns and cities. These developments would also affect our fellow organisms on the planet in a myriad of ways, both good and bad. Unfortunately, the list of negative effects we’ve had on other organisms is likely far longer than the list of positives. However, it seems safe to say that, at least up to a very recent point in our history, we had a relatively positive impact on the disease causing agents of this biosphere. First, we adopted a way of life that enhanced their ability to infect us, and then we traveled around the world, allowing them to spread with impunity through our vulnerable populations.

As mentioned earlier, humans lived as hunter-gatherers throughout most of evolutionary time. During this stage in our species’ existence, our pattern of disease must have resembled that of many other organisms. Humans at this time lived in small, mobile groups with a broad diet consisting of whatever could be found in the natural habitat. The broad nature of the human diet before the Agricultural Revolution resulted in essentially no malnutrition or deficiency diseases. Intestinal parasites are believed to be somewhat common among these early humans; however, their specificity to climate combined with the relatively static global movement of early human people limited the spread of such parasites (Ehrlich, 225).

Although labeled as “mobile,” the mobility of pre-agriculture humans was usually limited to movement within a particular region; larger scale movement across different regions also occurred, though much less frequently. The migration of Asian peoples across Northeast Asia and into the Americas, for example, could certainly be considered global movement of humans. Nevertheless, such movement was extremely gradual compared to the traveling people would be undertaking in an agriculturally introduced world.

The transition of human culture from hunter-gatherer societies to sedentary communities depending on the domestication of plants and animals for food was of great significance for many reasons. Ironically, although humans let go of the mobile way of living that had sustained them for so long to settle in permanent communities, this eventually brought about unprecedented human travel around the globe. This travel became an open door for disease to spread everywhere people did.

The first important consequence of “the adoption of agriculture and the transition to settled societies [was that it] exposed humans to a range of diseases they had never encountered before” (Ehrlich, 225). Disease spread rapidly through these settlements of dense human population; furthermore, the close proximity to which people now lived with the animals they had domesticated exposed them to a vast number of new infections. Smallpox, measles, tuberculosis, and diphtheria all originated from cattle. Additional infections include leprosy and the common cold which emerged from water buffalo and the horse, respectively. “[H]umans now share sixty-five diseases with dogs, fifty with cattle, forty-six with sheep and goats and forty-two with pigs” (Ehrlich, 226). The domestication of these and other animals or the lack thereof in different parts of the world would eventually pave the way for massive pandemics.

The advent of irrigation as an agricultural practice and the standing water it created provided another means for disease (mainly malaria and schistosomiasis) to infect people; ‘slash and burn’ agricultural techniques could also potentially create standing water (Ehrlich, 227). Agriculture generated food surpluses that had been unavailable from hunting and gathering and allowed human populations to grow rapidly. Overall, the increased exposure to disease failed to have any significant effect on the world population with the exception of pandemics that would plague various regions at various times. These outbreaks killed large fractions of regional populations until an immunity developed that allowed the population to rebound.

Finally, now that humans had made themselves vulnerable to a vast new range of new diseases, the complex culture developing after the Agricultural Revolution included various new aspects that would cause humans to travel across the entire world, bringing with them disease. The exploration, trade, and warfare of post-Agricultural Revolution humans would be the central cause for diseases traveling anywhere and everywhere. The diseases that the human species were susceptible to generally established themselves in several regions of the world. The Mediterranean and Near East, though hosting some diseases as individual regions, shared many because of extensive human contact through trade; the same was essentially true for China and India. On the other hand, the Americas, being entirely isolated from the Old World until the 1500s, shared a completely different pattern and variety of disease from everywhere else. Of course, this would eventually spell disaster for Native American people upon the arrival of Europeans.

Trade was the primary cause for the spread of many diseases in antiquity. The Mediterranean exchanged several diseases with India and China that infected and killed millions. “A sharp fall in the population of the Roman empire began with the great ‘plague’ of 165 AD, when the death toll was around a quarter of the population” (Ehrlich, 228). This particular outbreak is believed to have been caused by smallpox, a disease originating in India. China also suffered brutal death tolls from the introduction of smallpox from India between 150 and 300 AD. The bubonic plague also seemed to originate in India before spreading through trade to China and the Mediterranean in 542 AD. The resulting Black Plague ravaged all of China, Europe and the Mediterranean (Ehrlich, 228).

“The environmental conditions in armies with their overcrowding and primitive sanitation, made them not just one of the main suffers from disease but also one of the mechanisms for spreading it as they moved across the countryside” (Ehrlich, 232). Typhus was one of the main diseases that affected soldiers through history. “[B]rought back to western Europe by Spanish soldiers from Cyprus in 1490” and transmitted by lice, it was uncontrollable until the introduction of delousing stations in WWI (Ehrlich, 232). Disease often killed more men in war than the fighting did, and armies brought their infected with them to new, vulnerable populations as they traveled.

Lastly, human exploration served to take diseases to every corner of the world, most notably, the Americas. Europe had been exposed to and suffered from a variety of diseases for hundreds of years. The resulting pandemics killed millions; however, immunity to these diseases also developed among these same populations. By the 15 th century, Europeans exploring the New World carried the infectious agents with them to the Americas while being immune themselves.

Although diseases were exchanged both ways between Europe and the Americas, Native Americans suffered by far the worst consequences. This occurred for two main reasons. First, people in the Americas did not domesticate animals to the extent that the rest of the world did; this limited the transmission of infection between animals and people (Ehrlich, 226). Furthermore, although there were some mighty American civilizations possessing large cities, a hunter-gatherer way of life was considerably more common among Native American peoples than in Europe and other parts of the Old World. These factors contributed to the American populations not having to suffer many pandemics over time as the rest of the world had; however it also granted them no immunity and complete vulnerability to new diseases.

The first of these included smallpox, measles, typhus, and influenza, brought over initially by Spanish conquistadors during the 1500s. With no natural protection, the native population was decimated by these infections. Twenty-five million natives are believed to have lived in the valley of Mexico before European contact; by 1600, their number had dropped to around one million. Natives in North America suffered similar fates from the same diseases. The tropical region of Central and South America remained relatively unaffected until the introduction of malaria and yellow fever by the slave trade. Syphilis was the one American infection that traveled back to Europe with significant consequences. “Its effects were first widely noticed in the French army that invaded Italy in 1494 and from there it spread rapidly” to India, China, and Japan by 1505 (Ehrlich, 231). Although its infection was rampant, it failed to cause the massive death that was suffered by American peoples succumbing to newly introduced diseases.

The pattern of disease movement before the Agricultural Revolution seems relatively static when compared to that of later, agricultural societies. Trade and warfare spread infections across most of Asia, the Near East, and Europe; most notably, between the time of the Roman Empire and the 1800s. Furthermore, the European discovery and exploration of the Americas beginning around 1500 introduced the other side of the world to the cesspool of infections that had plagued Europe and Asia for thousands of years. From the 1800s on, tremendous improvements in both sanitation and medicine slowed the spread of disease around the world; however, the effort to contain and control the spread of infectious agents continues even today. Despite any new technology or medical advances, human travel will likely always be associated with the spread of disease.


Ponting, Clive. "Ch.11: The Changing Face of Death" from "A Green History of the World," St. Martins Press, NYC, 1991, pp. 224-239.


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last updated 5/13/06