“What Doesn’t Kill You: A Closer Look into the Black Plague”

Here is the latest winner of our “Student of Life” Award, bestowed on a student of BIOL 100 who writes a truly outstanding essay. The recipient for Spring 2018 is Katarina Milosevic, a talented junior with an interest in advertising and a self-declared “history buff.” Enjoy her thorough report on the Black Plague!


What Doesn’t Kill You: A Closer Look into the Black Plague

       The Black Plague, also known as Black Death, swept across Europe and the Mediterranean taking the lives of millions. First appearing in 1357, it lingered for centuries and ravaged through densely populated cities killing its host within the first few days of symptoms. Spreading like wildfire, it decimated entire families, villages, and towns leaving nothing behind but tragedy and shocking mortality. Killing one in every three Europeans, the pandemic petrified continents of people and nobody had an explanation for all the agony and suffering endured, until now. The following lays out the very framework that makes up the Black Plague: what it does, where it comes from, how it spread, of what it is made, and how its effects on the past directly explain important aspects of our current civilization. More importantly, it will prove how even only a few staggering genetic characteristics play an essential role in the makeup of all living things, including bacteria.

The Black Death arrived in Europe in October of 1347. Twelve “death ships” coming from the Black Sea docked at the Sicilian port of Messina. Almost all sailors on the ship were dead, while the few survivors were in critical condition and covered in black boils. Although turned away almost immediately by the Italians, it was already too late, the plague had found its next victims. Undeniably powerful and terrifyingly rapid, “people who were perfectly healthy when they went to bed at night could be dead by morning” (Black Death). Europe in the 14th century, hazy with war and poverty, welcomed the plague with open arms. Living in poor hygienic conditions, the typical human organism became easily susceptible to Black Death through food shortages, hunger, malnutrition and ergo, weak immune systems. The plague presented itself in three forms: the bubonic plague, the pneumonic plague, and the septicemic plague. The most common form, the bubonic plague, caused fever, chills, headaches, nausea, vomiting, swelling called buboes, and rashes all over the groin legs armpits and neck. The pneumonic plague largely focused on the lungs, producing a cough so severe an individual who contracted the disease was practically coughing up their own. Lastly, the septicemic plague, the vilest of the three, was the product of bacteria entering the bloodstream. Those infected suffered an incredible array of greatly unpleasant symptoms including abdominal pain, diarrhea, nausea, vomiting, fever, chills, weakness, bleeding (due to the inability of blood to clot), shock, and gangrene (black skin). Within just hours an individual could be in agony from a number of these symptoms, if not all of them. The Black Plague, in all forms, is a relatively fast death, but an astonishingly painful one.

Spread of bubonic plague by year (Nosikot)

The cause of Black Death wasn’t identified until the 1855 to 1959 outbreak in China when “doctors noticed that rats showed very similar plague symptoms to people, and that humans had flea bites” (Plague). Plague among people appears when rodents in human living quarters become contaminated. ‘House rats’ or ‘ship rats’, generally the black breed, prefer living near people. However, when these rats get infested and start dying off, their fleas turn to feed on humans. This results in the bubonic plague. For a disease to become an epidemic, it must be transferred to other rat colonies and local humans in the same way. Infectious bacteria can also “break out of buboes and be carried by the bloodstream to the lungs,” then are spread by cough and sneezes from one contaminated individual to another (Benedictow). A change in the way the disease is contracted creates another form of the plague, the pneumonic form. In this form of Black Death, “the lungs hemorrhage and the patient breathes out clouds of bacteria, spreading the plague directly from one human to another” (Scott). Nonetheless, the most common way the plague is spread is through rat fleas. While infected ship rats might have died, fleas survived, infiltrating cities where ships docked. They infest the clothing of any person they come in contact with easily invading one place to the next.


An important factor to consider when examining the spread of plague is the climate. “The fact that the plague is transmitted by rat fleas means plague is a disease of the warmer seasons, disappearing during the winter, or at least lose most of their powers of spread” however, “plague is very different from airborne contagious diseases, which are spread directly between people by droplets: these thrive in cold weather” (Scott). Black Death was able to thrive in all seasons due to its three forms. Two of which are contracted through rat fleas and one of which is airborne, allowing it to be present year round.

Rat Flea (McMahon)

That being said, there has been recent controversy on whether rats should carry all the blame for the development of the Black Plague. A recent study published in the journal Proceedings of the National Academy of Sciences (PNAS) instead proposes the idea that human parasites, such as body lice and fleas, could be the culprits of circulating Black Death. National Geographic’s “Maybe Rats Aren’t to Blame for the Black Death” outlines this possibility by explaining that the correlation of rat mortality to human mortality did not align: “medieval plague records don’t mention rats dying en masse” (Greshko). Samuel Cohn, a medieval historian at the University of Glasgow, disapproves of the rat-flea theory, “Geneticists and modern historians were putting the rat into the position [of spreading the plague] and were straining bits of evidence.” He says that the rats were wrongfully set up in this situation. The article also mentions that disease patterns in rat-flea versus human-parasite modes of transmission are mathematically different. After creating a virtual pandemic, where researchers simulated the rise and fall of an outbreak in accordance with several different European cities, they compared both rat-flea and human-parasite transmissions. After several replications, they found that 9 different European plague outbreaks best matched the human-parasite mode. Although a great breakthrough for rats, researchers still have a long way to go if they want to win this fight.


“Plague appears in a boom-and-bust cycle emerging suddenly to cause pandemics and then retreating” (Arnold). The outline of the plague’s genetic makeup uncovered that its ancestor was a practically innocent gastrointestinal pathogen called Yersinia pseudotuberculosis. This pathogen is only known to cause mild diarrhea and is so harmless that it can go unrecognized, while its descendant can kill an entire human organism in three short days. The main attribute that prompted Y. pseudotuberculosis’ successor, Yersinia pestis, to become so catastrophic was the attainment of a gene known as pla. Pla gave Y. pestis the power to cause pneumonia, composing one of the most infectious bacterium known and “producing a plague so lethal that it kills essentially all of those infected who don’t receive antibiotics” (Arnold). Y. pestis’ deadly success is also largely contributed to the structure of its DNA. It was found to contain a combination of plasmid, small circular DNA that replicates independently of chromosomes, and genome encoded components, such as pla, which allow it to efficiently infect a host. When Y. pestis and Y. pseudotuberculosis were compared, researchers found that a majority of the differences were neutral mutations, modifications that did not actively change Y. pestis’ characteristics. There are only a couple of differences that created such a mass contrast between the two bacteria. Firstly, Y. pestis has the ability to travel on fleas making it easier to spread. It is impossible for Y. pseudotuberculosis to do this because it carries a protein called urease giving fleas fatal diarrhea, while the genetic mutation in Y. pestis stops the creation of this protein. In fact, Y. pestis hardly gives fleas any illness at all. The second genetic change that allowed the plague to easily infiltrate the human body was found in Y. pestis’ ability to disarm blood clotting, one of the body’s most important defense mechanisms. The gene pla encodes a protein that dissolves blood clots, so when a flea pierces the skin, the blood cannot clot allowing the bacterium to rapidly spread within and make billions of replicas of itself. The pla gene is absolutely essential to the success of Y. pestis. Not only did Y. pestis obtain pla but it mutated it; “a chance mutation altered one amino acid in pla, greatly increasing its virulence by allowing the bacterium to penetrate more deeply into the body” (Arnold). Pla is considered as a “Big Bang event” in the plague, making it vastly invasive and twice as deadly. It is entirely fascinating how one small genetic change can create such a colossal difference in the entire architecture of an organism.

Circular representation of the Y. pestis genome (Parkhill, J. Nature News )

Even today, the Black Plague is still encountered every now and then. Although immensely less life-threatening than it was in the 14th century, without the treatment of antibiotics it still has the capability of killing an infected person in a few short days. In 2015, 2 hikers contracted the disease while camping in Yosemite National Park in California, and the last serious outbreak was reported in 2006 in the Democratic Republic of Congo when a minimum of 50 people died. “The United States, China, India, Vietnam, Mongolia, (and Africa) are among the other countries that have confirmed human plague cases in the recent years” (Plague). Preventive measures such as rodent and insect eradication, public health education, immunization, and genetic analysis of Y. pestis strains whenever found, are effective ways of keeping humanity guarded against the plague (Lotfy). Since it best thrives in crowded, dirty, rat-infested conditions, good sanitation and pest control in the home is a useful way to protect oneself and one’s family from Black Death (Plague).


In spite of the absolutely devastating effects it had on Europe and the Mediterranean, the ramifications of Black Death had an overwhelming and rather uncredited influence on European culture and history. Social, economic, and religious anarchy struck European cities as the plague reached its zenith of impact. Turning friends, neighbors, and even family members against one another, it constructed a social separation between the living, the sick, and the dead. England, one of many distressed countries, was at one point reported burying approximated 200 plague victims a day (Greshko).

The Triumph of Death (1562), by Pieter Bruegel Elder

After some of the foulest outbreaks, the social effects brought by the plague were felt almost instantly. Severe labor shortages leveraged poor serfs by releasing them from cruel contracts and giving them the freedom to choose whom to work for. Lords, who lived in fear of leaving their land unattended to, were forced to create better working conditions and offer better compensation; wages surpassed prices and therefore the standard of living improved (Riva). Years after the pandemic, empowered laborers revolted against elites when they attempted to return to pre-plague conditions. As a result, the middle class developed—to which the majority of Western Europeans belong today (Kochhar). Due to the potentially deadly fate of obtaining goods by trade, prices of local and imported goods rose steeply. Thus, the economy plummeted suffering sudden and extraordinary inflation. The slow collapse of wealth clouded the social hierarchy, which was primarily defined by financial gaps. In response, aristocrats sought to emphasize social status through expensive and flamboyant fashion styles. This attitude is still actively seen today by way of highly regarded fashion brands such as Fendi, Prada, and Chanel. Moreover, distrust in God and the Catholic Church soared when prayers went unanswered to stop the plague and one’s suffering. As priests died out, many church services ceased and people began to lash themselves begging God to spare them. This colossal defeat in faith later sparked the Protestant Reformation. Additionally, Jews were harassed and were said to have poisoned the water in wells that sparked the epidemic, making them to flee from Western Europe. The Black Plague likewise shaped English culture by unintentionally birthing the classic English pub. Because water was not considered safe to drink, weak beer became the drink of choice. In the face of despair, women continued to brew beers and began to open up their homes as public houses where people came to drink, eat, and socialize. “Subjected to unimaginable horror, people carried on and so the disaster was survived” (Robert Tombs). Interestingly enough, the trauma of Black Death left behind barely any devastating effects and predominantly had a positive influence on the construct of humanity. It revolutionized societal structure, recast economic distributions, redefined spiritualism, and inspired some of the most defining cultural practices.


To examine the past is to build a deep understanding of the effects of the present and foster the ability to predict events in the future. Science plays a vital role in this understanding. History may reflect today’s events and anticipate tomorrow’s, but science has the power of explaining why they happened and providing a solution. This topic resonates with me because I have always had a great interest in history and the insight it provides about society. Researching and studying the science behind the Black Plague, or Black Death, only gave me further insight into why it appeared in the first place. It opened up a whole new door to history I didn’t know existed and showcased how biology aids history in understanding humanity. It encourages me to take a greater interest in the scientific side of history and to continue feeding my thirst for knowledge in this subject with a wider approach. In my last paragraph, I spoke about the social, economic, and religious effects the Black Plague generated. I did this purposely to show how the two subjects go hand in hand. Biology set the roots of the paper while history expanded and pushed its boundaries.


Caused by a tiny mutation, the Black Plague wiped out millions, including 60% of Europe’s population. What started out as an ordinary stomach bug, grew into deadly bacteria because of only a few genetic changes. Yersinia pestis became the most infectious bacteria known by way of a single gene called pla. This gene gave it the capacity to cause pneumonia and ignite one of the most fatal epidemics seen by mankind. By discussing the fundamentals of the plague, the way it evolved, and its effects on society, simply the bare facts of the Black Plague depict the importance of genetics. The study of DNA and genes has become incredibly crucial to the survival of the human race. It helps us protect ourselves, improve ourselves, and above all understand the way we evolve. Although medicine has come a long way and antibiotics have greatly increased survival rates, it is daunting to think that a single mutation separates us from the next mass killer. Black Death has violently shown how even the smallest mutation can cause the biggest effect; how just a ripple in the water can cause a wave.

Katarina Milosevic

San Francisco, 6 May 2018


Works Cited

Arnold, Carrie. “The Mutant Genes Behind Black Death.” Quanta Magazine , The Simmons Foundation, 6 Oct. 2015.

Benetictow, Ole J. “The Black Death: The Greatest Catastrophe Ever.” History Today , History Today, 3 Mar. 2005.

“Black Death.” History.com , A&E Television Networks, 2010.

Editors of Encyclopaedia Britannica, The. “Black Death.” Encyclopædia Britannica , Encyclopædia Britannica, Inc., 5 Jan. 2018.

Greshko, Michael. “Maybe Rats Aren’t to Blame for the Black Death.” National Geographic , National Geographic Society, 16 Jan. 2018.

Kochhar, Rakesh. “1. The Middle Class Is Large in Many Western European Countries, but It Is Losing Ground in Places.” Global Attitudes Project , Pew Research Center, 24 Apr. 2017.

Lotfy, Wael M. “Plague in Egypt: Disease Biology, History, and Contemporary Analysis: A Minireview.” Advances in Pediatrics. , U.S. National Library of Medicine, 14 Nov. 2013.

McMahon, Dr. Harvey T. “How Yersinia Bacteria Cause Disease in Humans.” The Plague Black Death.

Moskowitz, Clara. “Black Death Was Selective.” LiveScience , Purch, 28 Jan. 2008.

Nosikot. “The West Has Decided on the Most Disastrous Period in the History of Europe.” Nosikot Live Journal , 1 Aug. 2016.

Parkhill, J., et al. “Genome Sequence of Yersinia Pestis, the Causative Agent of Plague.” Nature News , Nature Publishing Group, 4 Oct. 2001.

Perry, Kellen. “14 Ways the Black Death Directly Shaped the Way We Live Now.” Ranker , Ranker, 2016.

“Plague.” National Geographic , National Geographic Society, 18 Jan. 2017.

Riva, Massima. “Social and Economic Effects of the Plague.” Decameron Web , Brown University, 12 Mar. 2010.

Scott, Susan, and Christopher J. Duncan. “The Black Death: Was It Caused by Rats or Viruses?” Times Higher Education (THE) , Cambridge University Press, 22 May 2015.

Wilkes, David. “How Black Death Drove Survivors down the Pub.” Daily Mail Online , Associated Newspapers, 25 June 2015.

Williamson, E D, and P C F Oyston. “Protecting against Plague: toward a next Generation Vaccine.” Advances in Pediatrics. , U.S. National Library of Medicine, 11 Mar. 2013.

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