Many diseases have affected the outcomes of battles or the political leanings of a country, but few have had consequences on society that continue to be felt in the present age. The plague is one such disease and its most famous pandemic – the Black Death – has changed the history, culture, and science of Western society.

In July 2014, Chinese officials shut down entire sections of the city of Yumen after a 38-year-old farmer fell ill. His symptoms and history: a fever and contact with a dead marmot. His diagnosis: bubonic plague [1].

For most people, the plague lives in the pages of medieval history books under the ominous name “Black Death,” which refers to the plague’s most famous outbreak. Despite its ties to the middle ages, the plague is alive and well in the 21st century. The World Health Organization (WHO) reports between 1,000 and 2,000 cases yearly, the majority of which occur in agricultural areas of Africa [2]. Closer to home, the bacterium that causes plague, Yersinia pestis, is endemic to certain areas of the southwestern United States and causes a few isolated cases every year [3]. Despite its name, the disease responsible for the Black Death is currently treatable with easily attainable antibiotics and death only occurs when treatment is delayed.

The plague is so prevalent in European history textbooks because, more than any other disease, it has had a heavy hand in shaping European society. Furthermore, in dealing with the Black Death, European governments brought about some of the first instances of concerted public health measures, many of which have been proposed for dealing with the current Ebola crisis.

The Plague

The causative agent of the plague is Yersinia pestis, a bacterial species that is mostly carried by rodents and transmitted between them by fleas. In the words of Robert Koch — the microbiologist who, in the late 1800s, developed the theory that microorganisms cause disease — plague is “a disease of rats in which men participate” [4]. In other words, humans are not a normal host of Y. pestis and are only occasionally infected. When an unlucky human does catch plague, it is mainly through flea bites. Most infected people suffer from bubonic plague, the most common manifestation of plague (Figure 1).

Bubonic plague begins with a fever, general malaise, and a blister at the site of infection. Once the bacterium has invaded the lymph system — the drainage and defense system of the body — it arrives at a lymph node, causing it to swell to about the size of an orange. The swollen lymph node is called the bubo, which is the most distinctive feature of bubonic plague. Finally, the infected individual begins to bleed internally and externally, and in some very extreme cases the patient may have neurological damage or enter a coma.

Figure 1 Yersinia pestis’ animal cycle involves primarily fleas and rats. When an infected flea finds a new rat host, it transfers Y. pestis to that rat. The infection eventually causes the death of the rat, dislodging the flea who is forced to seek a new home. Normally the flea jumps to a new, uninfected rat, but occasionally it ends up on a human, where it can transmit Y. pestis to its human host. In most cases, infected humans will have the bubonic form of plague: Y. pestis will enter the lymph system and cause lymph glands to become swollen (Image courtesy of the Centers for Disease Control and Prevention’s Public Health Image Library [8]). Eventually the disease progresses to internal bleeding and death if untreated.

Pestiferous Mortality: The Black Death in Europe

Movies and books about pandemics may seem to be a current phenomenon, but their origin can be traced back to the 1350s when Giovanni Boccaccio published The Decameron. Boccaccio’s work describes the story of ten people who leave Florence while the city is in the grips of the Black Death and seek the safety of the countryside, where they pass the time by sharing stories.

The ten fiorentinos were correct in fearing this “pestiferous mortality” [5]. The Black Death began in central Asia in the 1330s before spreading to Europe and the Middle East through trade routes (Figure 2). It arrived in Italy aboard trade ships from Crimea and from there spread to the rest of Europe, killing one-third of the European population along the way [4]. The Black Death would not have achieved pandemic proportions without commerce and intense trade between Europe and Asia.

Europeans in the Middle Ages were no strangers to disease and death. Medicine still relied on the concept of Greek humors (a now discredited idea that an individual’s health was controlled by four bodily fluids, or “humors”), and the average life expectancy was only 35 years. But the plague’s extreme virulence, high case fatality, and ability to strike even the healthiest individuals maximized the terror surrounding it, causing hysteria, religious revival, and a mass exodus of individuals from affected cities [6].

To make matters worse, many of the symptoms of the plague were particularly demeaning. Reports from the time speak of an overwhelming stench that appeared to emanate from the affected [4]. Furthermore, because Europeans believed the plague was spread by miasma, or “poisoned air,” the stench was thought to be contagious, and individuals either fled from the sick or took to wearing elaborate, menacing masks. As such, death was a terrifying, revolting affair. Many suffered alone as family members and friends abandoned them in a desperate attempt to save themselves.

The horror of the Black Death did more than create the genre of plague fiction. Symbols of death – the grim reaper, scythes, skeletons – flourished in paintings and other art forms. The deaths of millions by the same affliction also contradicted the established medical belief that humors determined an individual’s health since it was surprising that millions would simultaneously suffer from the same fluid imbalance. This made way for the theory of contagion – the idea that disease was spread by chemicals – that would prevail until Koch’s germ theory of the late 1800s. But, most importantly, it brought about the first government-sponsored efforts to control and eradicate the disease.

Figure 2. Timeline of the 2nd plague pandemic (1330s-1830s).

The birth of public health

Although established medical opinion continued to hold on to humoral medicine until Girolamo Fracastoro published his theory on contagion in 1546, the public health measures adopted during the Black Death reflected the layman’s correct assumption that plague was contagious [6]. These measures were first adopted in Italy and later spread to the rest of Europe and involved some of the first instances of quarantine and lazarettos.

Quarantine is a government-imposed isolation of an individual or group of individuals who may be contagious for a given disease. Lazarettos were houses set up to isolate both the plague-stricken as well as suspected cases. The public health measures focused heavily on restricting new infections. In many cities there were compulsory burials, possessions of the recently deceased were burned, and cordon sanitaires, barriers to limit overland movement, were established. To enforce these measures, local governments came to depend on the military [6].

In terms of movement by sea, empirical observation had shown that in many coastal towns the plague tended to arrive by ship. In Daniel Defoe’s A Journal of the Plague Year (1722), a fiction/non-fiction hybrid about the Great Plague of London (1665), Defoe mentions that “the trading nations of Europe were all afraid of us; no port of France, or Holland, or Spain, or Italy would admit our ships” [7]. Ports turned away ships or established an unnecessarily long 40-day maritime quarantine, at the end of which anyone on board who had caught the plague – flea, rat, or human – would have either survived or perished [6].

While these public health decisions were made without a clear understanding of the biology of the plague, they were eventually successful, particularly after the military became involved. On the other hand, the quarantines, the lazarettos, and the boarding up of houses in which plague-stricken individuals were dying were incredibly frightening, draconian measures that oftentimes prompted people to flee and, as such, probably served to increase the spread of the plague [6].


When a disease doesn’t yet have an effective cure, public health measures are sometimes the only action governments can take. The global fear over Ebola that has gripped the world in 2014 gives us an idea of what medieval Europeans were dealing with during the height of the Black Death. Many of the measures proposed for dealing with the Ebola crisis – quarantines, travel restrictions, etc. – have their origin in the 2nd plague pandemic.

Of course, there has been much progress since the Medieval Ages: scientists know the causal agent and means of transmission of Ebola and they have history on their side to tell them which measures do more harm than good. So, unlike the blundering Italian government holding bonfires to purify the air of any poisonous chemicals or enforcing 40-day quarantines, for Ebola, we have a scientifically sound 21 days of observation and very clear and informed safety recommendations from the government agencies responsible for dealing with public health. But public heath measures adopted during the Black Death, and now again for Ebola, have the same guiding principle: to limit the number of new infections. We should take heart from their ultimate success with plague.

Fernanda Ferreira is a 2nd year graduate student in the Virology Program at HMS (Harvard Medical School).


[1] Jonathan Kaiman (2014). In China, a single plague death means an entire city quarantined. The Guardian. Retrieved from

[2] World Health Organization (2014). Health Topics: Plague.

[3] Centers for Disease Control and Prevention (2012). Plague.

[4] Kelly, J. (2005). The Great Mortality: an intimate history of the Black Death, the most devastating plague of all time. New York, NY: Harper Collins.

[5] Boccaccio, G. (2007). The Decameron of Giovanni Boccaccio.

[6] Snowden, F. (2010). Plague I-III. Hist 234: Epidemics in Western Society since 1600s. Lecture conducted from Yale University, New Haven, CT.

[7] Defoe, D. (2006) A Journal of the Plague Year.

[8] Centers for Disease Control and Prevention (2005). Public Health Image Library (PHIL) ID# 2047.

14 thoughts on “Plagues of the Past

  1. oh my gosh what an amazing website, i learned so much from this.
    thank you Harvard university for making this.

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