What were the Main Causes and Consequences Concerning the Spread of Epidemics in the Past?

Rebecca Gaston
University of Aberdeen
r.gaston.17 [at] aberdeen.ac.uk

The main causes regarding the spread of diseases in the past are the expansion of trade network and the increasing practice of colonialism. Selective pressures from the host on the pathogen cause it to evolve certain aspects to aid in reproduction and the passing on of its genetics. This makes it extremely dangerous to individuals who have not encountered it before and lack the natural resistance that would have been created with previous contact to the disease. Trading and colonialism caused different groups to come into contact with each other, causing the widespread infection of diseases which eventually caused epidemics. This killed millions in some cases, one example being the bubonic plague outbreak of the 15th century that resulted in approximately thirty to forty million deaths in Europe alone.  


A disease is an abnormal medical condition affecting the body of an organism that exhibits a specific number of symptoms belonging to the particular disease (Jones 2019). There are many types of communicable diseases, such as the bubonic plague, small pox and yellow-fever, that can eventually cause the deaths of thousands of people if left uncontrolled. Fortunately, due to recent advanced medical technologies and medication, such as vaccines and more recently, the first anti-viral treatment for smallpox called tecovirimat (TPOXX) (Mucker et al 2013; p.6246). These diseases can be contained and treated. However, in the past, this was not the case, and throughout history there are many horrific examples of epidemics that occurred across the globe. They were able to spread due to many reasons; the main ones being the expansion of the trade network, in which traders would either directly carry the disease or pick up pests, like rodents, and bring them back with them, and the colonialisation of ‘newly discovered’ lands, resulting in the thousands of deaths of indigenous people due to their lack of natural resistance. A lack of knowledge concerning the diseases and its treatment also aided in the rapid diffusion, especially in indigenous communities where they would easily catch the diseases due to no immunity and have had no contact with many deadly diseases, therefore having no prior knowledge to help regulate the epidemic.

The Role of Trade

The expansion of the trade network was hugely beneficial to many economies and communities in numerous ways: It increased employment rates, introduced new technologies, furthered knowledge on foreign societies and cultures, ended conflicts and allowed new relationships to form between countries separated by hundreds of miles (Grossman & Helpman 1990: 1-2). However, goods and services were not the only things that was traded. There are many instances where a trading party would be wiped out by a mysterious illness in only a matter of days, leaving people baffled due to the lack of knowledge of the strange illness. Humans and pathogens exert selective pressures on each other, resulting in the pathogens evolving and adapting depending in the pressures applied on them by the host they are infecting. If the pathogen is too deadly, then it doesn’t have enough time to multiply and infect other hosts fast enough, also, they adapt a resistance to treatments to eradicate them (Jones 2019). Similarly, the pathogen causes the host to adapt by removing certain alleles that have had previous exposure to a disease, so they cannot reproduce and pass of the genetics of said disease. Alleles that were able to survive the disease can pass on their genetics to offspring leading to resistance in human groups (ibid). From this we can deduce that individuals who have never been exposed to certain diseases would not have the correct alleles for natural resistance and certain cells called Killer Cells that provide resistance to certain diseases (Herberman & Ortaldo, 1981: 24-26). They would be infected by the highly evolved pathogen, not having evolved to be able to fight it off, resulting in an extremely severe, assured and fast death. A trader may go all over the world and through many countries that all have varying types of communicable diseases, contract them and, without realising the implications, take this disease back to their homeland where others would then contract the same illness, spreading it.

There have been three outbreaks, in which it reached pandemic proportions, of the bubonic plague. The epidemic of the 14th century was the most devastating in terms of the number of deaths (McEvedy 1988: 2). It was dubbed ‘The Black Death’ due to its distinctive symptoms including buboes (painful and swollen lymph nodes usually found under the arms, neck and groin area), that would ooze ‘black’ blood giving the plague its famous name, skin rashes, acute fever, the vomiting of blood (Poland et al. 1973: 332-334). It is extremely contagious, transferred by a bacterium called Yersinia pestis, spread by fleas and lice on people and animals. It is also possible to be transferred directly from infected animals and people. Once contracted, death usually occurs in 2 to 7 days (ibid).

This plague outbreak is a prime example of how major the role of trade was in the rapid and extensive spreading of the Bubonic plague from Eastern Europe, across central Europe and eventually into Great Britain. Evidence suggests that the plague originated in China and was brought into Crimea by traders in 1346, this spread to Italy and southern France in 1348, causing mass devastation before moving to Great Britain and eventually Scandinavia by Bergen and through trade routes from Russia (Benedict, 1996: 1). Early accounts detail many deaths along popular trade routes such as the Silk Road and trade routes from Scandinavia, the Baltic, the North Sea, the Atlantic coast and the Mediterranean, including river trading into England, carrying the plague without check (Zeigler, 2013). It reached England in 1348, through Devon Port, and Scotland by 1350, moved by trade and passenger ships, carrying rodents with fleas infected by the deadly bacteria (ibid; Harrox 2013).

The Black Death caused devastating consequences for Europe with the death of approximately 40 million people in total and 45-50% of the population of Great Britain wiped out, there were major social and economic impacts that caused even more turmoil. Due to the extremely high number of deaths, there was not enough people to bury the corpses, resulting in more people becoming infected due to exposed disease-ridden fluids from decomposition and an increase in rodent activity. As a way of coping with the influx of bodies, mass graves were established. Two examples of which can be found in Provence, Southern France, where testing was done on molar teeth to find out if Yersinia pestis can be detected in human tooth pulp (Drancourt et al., 1998: 12637-12638). The results showed that these mass graves were filled with Victims of this bacterial disease. Another consequence of the plague was that there were not enough labourers to work the land and produce food. As a result, there was widespread famine, causing many more deaths, especially individuals already weak from sickness. Many demesne farmers had to fight and protect themselves from the high, and occasionally violent, demands of workers for higher wages, that would threaten the economy of their farming estates (Bridbury, 1973: 585). High pressure and stress situations can cause people to act out and blame different people, this started the high level of persecution of certain groups of individuals. Especially anti-sematic persecution, where many Jewish people were being flogged, beaten and even burned alive as a religious discipline in Germany (Zeigler, 2012: 2; Cohn, 2007: 3-36).

However, there were many places that took great precaution to prevent and control the spreading sickness; Milan is one of these places. Luchino Visconti quarantined the city, not allowing many from the outside in unless proved they didn’t have the plague (Carmichael 1991: 215). He also ordered that any house with cases of the plague, were to be completely sealed up, isolating the sick from the healthy (Mount, 2017).  This meant that they were not as affected by the eventual impacts from the outbreak as other places. However, Milan is only one city in Italy, the rest of the country was completely overcome with the disease resulting in hardly any difference in the aftermath.

Another example of how trade was largely responsible for the spreading of epidemics is the great south-eastern smallpox outbreak in the 17th and 18th Century (Kelton 2002: 21). Smallpox is thought to have originated in Africa, having evolved from an orthopoxvirus of animals in the rain forest and first became a virus specific for humans in the Nile Valley approximately five thousand years ago, then spreading gradually into Europe (Fenner 1993: 6). However, once again due to the rapid expansion of trade in the late 1600s, the disease was able to move rapidly and unhindered into Native American populations in South-eastern America through trading ships by the infected crew and animals. Without previous exposure to the illness, the indigenous populations were extremely vulnerable to infection and more violent symptoms. As a result, the epidemic quickly spread and grew into a catastrophic event, producing a high number of deaths (Jones and Jones 2009: 68-70). Not all the deaths were caused by the disease, many were in fact caused by the lack of people to collect necessities such as food and water, or no one to protect the settlement from enemy attackers. This meant many people died from starvation, dehydration and conflict issues (Kelton, 2002: 21-26).

The most damaging cause of the diffusion of smallpox was the acceleration of the Indian and African slave trade in the late 17th century (Patterson and Runge, 2002). Many of the slaves and traders were infected with the disease and spread it everywhere they went, originating in Virginia going south into Carolina, into central North America, and all the way to the Mississippi River, where it wiped out many Aboriginal communities (Kelton 2002: 22).  

The Role of Colonialization:

Colonialism is a form of imperialism in which a country gains power by taking the subjugation a nation to gain control of other’s land and goods to use as their own (Lydon and Rizvi, 2010: 20) (Kohn and Reddy, 2006). This is usually done through missionary and militaristic means (Alfred and Corntassel, 2002: 597-598). Colonialism developed in the 15th century, which was dubbed ‘The Age of Discovery’ due to the increased level of exploration and discovery of ‘new’ lands due to the technological developments in navigational equipment. Faster sailing ships allowed colonists to reach distant ports faster, using less resources and enabled relationships to be sustained between the colonising power and the colonised (Kohn and Reddy, 2006). This mostly exercised by the Spanish, Portuguese, French, English and Dutch initially. As it was with trade, European diseases and illnesses were carried to these lands by the colonisers or the animals in the cargo and infected the animals and indigenous people that had never encounter them before, wiping out whole communities of people within a few weeks due to no natural resistance. To the native people, even the common cold can be deadly.

An example of this can be seen in the explorations and colonisations of Christopher Columbus, who, on his first voyages, set up settlements on many of the islands establishing the, now named, Caribbean Islands (Wilson 1993: 38). However, the main damage was done on his exploration of 1493, his third and final voyage. At the beginning of this journey there were 1500 Spaniards on a floating flotilla from Cadiz in Spain. Many animals were also taken on this expedition. En route, many sailors fell ill, including Columbus and several of them died, giving an indication of some form of disease they were carrying that was serious enough to cause death in some cases. Adverse winds caused the fleet to be carried to an Island called Ayti, now called Haiti. Columbus renamed it La Isla de Española (Britannica, 2019). With the help of the Taino Cacique, a stockade was constructed. It was in 1493 that huge numbers of the local people began falling sick and dying with the symptoms like that of Influenza and Smallpox (Guerra 1988: 305-307). This illness was passed on by the colonisers and Columbus, as they suffered from it too, but survived long enough to infect the indigenous people.

The consequences of the outbreak were devastating. In 1492, before the arrival of the disease, the population of Taino was 250’000 and in 1517, the population was only 14’000 showing a dramatic decrease in a short time period (Stoneking 2009: 1-2; Henige, 1986). Henige concluded that in 1518, smallpox epidemic had wiped out the Taino (ibid; Dobyns, 1993: 277). However, the number of deaths totals considerably more. The native populations moved between islands regularly for labour, aiding the rapid spreading of the disease and increasing infection (Stoneking 2009: 18-19). The animals that were brought over by Columbus were also thought to have been infected and responsible for transferring some of the diseases, such as Swine Influenza, transferred and carried by pigs (Jones, 2019).  Overall, the Disease was also too deadly for even Columbus’ group as the original 1500 individuals in 1493 decreased down to only 300 in 1502.

As mentioned previously, another example of how colonisation is one of the main causes concerning the spread of epidemics is the smallpox epidemic brought directly into Native American populations by European settlers. One of the first European settlement established was Jamestown in 1607 on the bank of the James River (Kelso, 2006). After it was founded, it was easier for other travellers and traders to enter America, increasing the likelihood of disease being introduced to the area. Inevitably, successive waves of epidemics occurred, the two most notable in 1775-1782 and 1837-1838. The first of which was caused directly by Europeans settlers interacting with the vulnerable native Americans to take over their lands in the Hudson Bay region (Carlos and Lewis 2012: 278). Trade was also a large factor, especially due to the largely expanding fur trade causing the Natives and the settlers to interact regularly. Interaction and the spreading of Smallpox was more easily spread by the advancement in transport, specifically the use of horses, reintroduced by Columbus (Cortés et al., 2016: 341) and river transport. As a result, thousands of natives were infected and killed by the disease. In the Valley of Mexico approximately 22’000 natives were killed. It reached pueblos of New Mexico by 1780, killing 5’000 and in total approximately 130’000 people, the majority of which were indigenous populations (Fenn 2001). Overall causing a mortality of at least 50% of native populations in early estimates and later, 75% to 95% (Carlos and Lewis 2012: 277).

The Natives were affected badly, not only because of no natural resistance, but also a lack of knowledge regarding treatments, and, in some cases, their traditional treatments caused the symptoms to become worse, hastening and increasing deaths. In their communities, the Native Americans believe in community care causing the infected to encounter large groups of people, possibly infecting them as a result. Treatments that were practiced could be damaging to the infected, weakening them further. This included sweat lodges and cold emersion baths (Portman and Garrett, 2006: 458). A lack of vaccination programmes for smallpox and influenza enabled the sickness to spread unchecked and uncontrolled.

Intentional Infection: Colonial Warfare

Figure 1: A depiction of The Death of Jane McCrea by John Vanderlyn painted in 1804
Figure 1: A depiction of The Death of Jane McCrea by John Vanderlyn painted in 1804


The Indigenous people were always considered inferior to the European settlers. Due to their cultural differences they were seen as savages and uncivilised, therefore deemed as outcasts and people not to interact with. For example, when Columbus first encountered the Native Americans, he only saw them as another specimen to study and add to his collections (. He used his knowledge the eclipse to exert power over them and to him, because he was able to do this, it proved that they were not human beings just another species of wild animal (Keal, 2003: 58). The painting, The Death of Jane McCrea by John Vanderlyn, is an interpretation of the murder of Jane McCrea who was supposedly killed by Indian scouts. The painting was commissioned to compliment a poem, and, in this poem, it focuses on the innocence of the woman and the brutality of the Indians saying they had a “demon grin” when killing her (Sheardy, 1999). The painting was to highlight the savage nature of the natives in the killing of an innocent woman. However, many believed it to be a cover up with her real cause of death being due to an accidental shooting and used as propaganda against the Indians (Strange Remains, 2015).

In many instances there were conflicts between the Europeans settlers and the natives due to mutual ideas of superiority and proprietary of the land. These struggles usually resulted in more indigenous deaths due to their less advanced weaponry and protection. This conflict commenced in the early 17th century, specifically in 1622 when the Powhatan Confederacy nearly killed the entire population of the newly established Jamestown (Powell, 1958: 44-46). As a result, the settlers retaliated and thus began the American-Indian Wars spanning from 1622-1680, starting with the Virginia massacre of 1622 (Vaughan 1978: 57).  

In order to be victorious over the Indians, they used certain techniques that could be interpreted as the first instances in the development of biological weaponry in colonial warfare (Riedel 2005: 21). In many documented incidents, it was implied that the settlers intentionally infected the Native Americans with Smallpox to wipe out large populations of them to control and exert power over them (Alvarez, 2014: 86-91). It originated as an idea from Francis Parkman during the rebellion of 1763 who asked, “could it not be contrived to send the Smallpox among those disaffected tribes of Indians?” (Ranlet, 2000). Such an event was orchestrated by William Trent on June 24th, 1763, gifting blankets and handkerchiefs from the Smallpox hospital on his ship as a sign of ‘good faith’ and then stating, “I hope it will have the desired effect” (Ranlet, 2000). It was later documented that Smallpox did indeed spread to the Indians, killing hundreds and wiping out entire communities (Valencia-Weber 2002: 407). An early example of genocide.


Due to the selective pressures on a pathogen and its host, they are both forced to evolve. The pathogen evolves so it can survive longer in the host to reproduce and pass of its genetics, making it deadlier. The host must eradicate the disease to be able to pass on the alleles that have become resistant to the disease to its offspring. The evolution of the pathogen was what caused millions of deaths to occurs due to a lack of natural resistance created from encountering the pathogen in the past. Therefore, if a disease comes from an area that has had no prior contact to the visitors, each party is vulnerable to contracting an unknown disease and in turn infecting other people, creating an epidemic.

The main two reasons for the spreading of epidemics in the past are: The rapid expansion of the trade network and the increasing practice of colonialism. Traders brought diseases to new lands and infected the indigenous people there, wiping them out within a few weeks due to a lack of resistance to the pathogen. They could also be infected by indigenous people and then take the disease back to their homeland, starting an epidemic there. This was the case regarding the Black Death originating in China and eventually reaching Europe through trade routes. Colonialism also caused many epidemics within the Native peoples as can be seen from Columbus’ colonialist expedition when they caused an outbreak of what is thought to be Smallpox or Influenza, killing hundreds of thousands of Native Americans across the country. Colonial warfare in the 17th century saw the first instances of biological weaponry being used in the form of the intentional infection of Smallpox in Native populations to gain power and control over them in the Indian-American wars, killing thousands as a result.


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