The Black Death, one of the deadliest pandemics in recorded history, devastated Europe, Asia, and Africa at the end of the 14th century. The loss of life was staggering — estimates suggest anywhere from 30 to 50 percent of all Europeans died. It was not a result of any kind of supernatural force or curse, however. Instead, it was the result of the actions of a very specific bacterium, transmitted mainly by fleas traveling on rats. To understand the plague, we need to know what caused it, where it came from, and what allowed it to be so deadly.
The Cause: Yersinia Pestis
The bacterium responsible for the Black Death was identified as *Yersinia pestis* . *Yersinia pestis* is a rod-shaped bacterium. It was first described in 1894 by French physician Alexandre Yersin after he found the bacterium present in the bodies of those killed in the Hong Kong plague outbreak. Although Yersin worked separately of Japan’s Kitasato Shibasaburo, both scientists discovered *Yersinia pestis* , and Yersin was able to link the bacterium to the plague. This connection helped to establish a biological basis for the disease. Historically, prior to Yersin’s discovery, people believed the plague was due to various causes including astrology and divine retribution. By linking *Yersinia pestis* to the plague, Yersin established a biological reason. Today, scientists use genetic analysis to confirm that the strain of *Yersinia pestis* responsible for the medieval Black Death is genetically similar to, although distinct from, current plague isolates. For instance, researchers used genetic sequencing to identify parts of the genome of *Yersinia pestis* obtained from individuals buried in mass graves associated with the Black Death. This provides genetic evidence that *Yersinia pestis* caused the plague. A study published in *Nature Genetics* in 2011 utilized this method to obtain evidence of the role of *Yersinia pestis* in causing the Black Death.
Transmission
The typical model of how the Black Death was transmitted describes a three-part relationship among *Yersinia pestis*, fleas, and rats. The primary vectors were Oriental rat fleas (*Xenopsylla cheopis*) living on black rats (*Rattus rattus*). Once an Oriental rat flea fed on an infected rat, it ingested blood containing *Yersinia pestis*. After feeding on an infected rat, the *Yersinia pestis*-infected flea feeds again on another animal (often a human) and vomits up the bacteria into the new host’s bloodstream. As long as a flea has a clogged digestive tract filled with *Yersinia pestis*, the flea will continue to feed because it is always hungry. Therefore, fleas carrying *Yersinia pestis* will repeatedly jump off dead or dying rats onto human hosts. Typically, humans contracted the bubonic form of the plague by being bitten by fleas jumping from dying rats onto human hosts. However, in addition to rats and fleas acting as primary vectors, humans–to–humans contact also facilitated transmission, particularly in cases involving pneumonic plague and/or septicemic plague. Pneumonic plague occurs when *Yersinia pestis* infects the lungs. The disease can be transmitted person-to-person through airborne droplets produced by coughing or sneezing. Septicemic plague occurs when *Yersinia pestis* multiplies within the blood stream producing systemic infections. Septicemic plague may develop from either bubonic or primary infections with *Yersinia pestis*.
Origin
Genetic data and historical records point to Central Asia as the geographic region from which *Yersinia pestis* emerged as a pandemic agent. Researchers hypothesize that *Yersinia pestis* evolved from wild rodent populations (such as marmots) located in Central Asia (presumably in the steppes around Tian Shan Mountains) that serve as natural reservoirs for *Yersinia pestis*. From these endemic areas, *Yersinia pestis* is thought to have been transported along trade routes (the Silk Road). Migrants and traders transporting goods frequently brought with them rats and their fleas. These same animals unknowingly carried *Yersinia pestis* to other regions.
European Route
Trade is considered one of two routes that lead to the introduction of *Yersinia pestis* to Europe. One possible entry route was through Crimea. In 1346, the Mongol Golden Horde, under Jani Beg, laid siege to Genoese trading port of Kaffa (Feodosia today), situated on Black Sea coast. Accounts of the time provide descriptions of how Mongol forces catapulted diseased cadavers into Kaffa. Whether or not such events occurred with great frequency remains disputed among historians. However, it is known that *Yersinia pestis* entered Kaffa. Genoese sailors fled Kaffa to escape the Mongol invasion and took ships that carried *Yersinia pestis* into ports throughout the Mediterranean (notably Constantinople, Sicilian port city of Messina and Marseille). From those ports, it spread rapidly into interior of Europe.
Why Was It So Deadly?
There were several contributing factors to why *Yersinia pestis*, carried by fleas on rats, proved so fatal:
High Population Density: Cities in Europe were extremely populated and poorly sanitary in 14th century. Due to proximity of living spaces, diseases could spread quickly.
Poor Sanitation: Unsanitary waste management systems enabled large numbers of rats to thrive.
Malnutrition/Famine: Europe suffered famine for decades preceding the Black Death (for example, 1315-1317 famine), which left people vulnerable to illness.
Lack of Medical Understanding: Physicians knew nothing about bacterial agents or treatments for them at that time. They treated illnesses using treatments based upon superstition (including bloodletting) and combinations of herbs.
Trade Networks: Extensive networks of trade developed in part to facilitate commerce; they served as highways for pathogens.
Legacy
The consequences of the Black Death continued to affect European society for generations. The sudden availability of labor due to high mortality rates created an environment in which laborers enjoyed greater bargaining power than ever before. Labor shortages resulted in rising land prices and increasing wages for laborers. Additionally, many saw an opportunity to question authority and challenge social norms in times of great upheaval. In response to the trauma experienced during the plague, artists expressed ideas centered on death and suffering. Additionally, the plague prompted advances in public health; quarantine stations and improved sanitation facilities began appearing in some cities.
The plague continues to appear periodically since then, killing millions each time; therefore, it has become a global concern regarding biology and history alike.
