Internet Archive’s copy of Eugen Holländer, Die Karikatur und Satire in der Medizin: Medico-Kunsthistorische Studie von Professor Dr. Eugen Holländer, 2nd edn (Stuttgart:Ferdinand Enke, 1921), fig. 79 (p. 171).

Whether the iconic, mysterious plague mask worn by medieval doctors or the rats and fleas that are the culprits in spreading the disease come to mind, many mysteries and myths persist to this day. The plague had a profound effect on human history, shaping demographics, economies and societies.

Today we’ll dive into some history, dispel some myths you may be carrying about the black death and discuss today’s leading research and theories behind the origins of the infamous epidemic. 

Bubonic plague: myth and differing origins

The plague has struck multiple times throughout history. One of the first recorded major outbreaks struck the Byzantine empire and is known as the Plague Of Justinian (541-542 AD), estimated to have killed about 25 to 50 million people during Emperor Justinian I’s rule. The most infamous outbreak of course was the one in the mid-14th century which resulted in the death of an estimated 25-30 million people in Europe (1347-1351). The plague killed more than 33% of the population in Europe during the 14th century within a few years. Although treatable through antibiotics, according to the World Health Organization (WHO) there are still approximately 1,000 to 2,000 cases yearly.

The Bubonic plague, often referred to as the Black Death when discussing the famous medieval pandemic outbreak, is a highly infectious disease caused by the bacterium Yersinia pestis, which can be cured when promptly treated. The Bacterium was named after Alexandre Yersin, the scientist who identified the bacterium during the third pandemic (1855-1960) in China. 

However, this finding was only made in the 19th century with advancements in bacteriology and through Yersin’s meticulous observations, giving human beings increased agency and understanding around epidemics and the spreading of diseases. It was then identified that this bacteria would spread from rats and fleas to humans, which was the primary mode of spreading during the epidemics. 

Before Yersinias observations and improvements in bacteriology, and the findings that associated rats and fleas, many theories were proposed to explain the origins and transmission of the plague. These theories were often based on the limited scientific knowledge of the time reflecting cultural, religious, and medical beliefs. Sting theories about the origins of the epidemic have floated around through history. 

Historical theories

Miasma theory

The most prevalent theory was the miasma theory, which suggested that diseases, including the plague, were caused by "miasmas" or "bad air" emanating from decomposing organic matter. People believed the foul-smelling air from swamps, garbage, and decaying bodies carried poisonous vapours that caused illness.

To combat miasmas, people used aromatic substances such as herbs, flowers, and spices to purify the air. Plague doctors' masks, with their long beaks stuffed with these substances, were based on this theory.

Contagion theory

Some doctors and scholars believed the plague had spread through direct contact with infected people or their belongings. This led to practices like isolating the sick, quarantining ships and travellers, and fumigating houses and goods. There was also a belief that the plague could be transmitted via contaminated clothing and bedding, leading to efforts to disinfect or destroy such items.

Religious and supernatural theories

Many people saw the plague as a punishment from God for the sins of humanity. This belief led to religious processions, prayers and penance rituals aimed at appeasing divine wrath.

Some attributed the plague to unfavourable astrological alignments or celestial events, such as comets or planetary conjunctions, which were thought to influence earthly affairs.

In some areas even, people accused minority groups of poisoning wells or spreading the disease through witchcraft. These accusations led to violent persecutions and pogroms. During wartime, sometimes the theories floating around enemy enemies deliberately poisoning wells circulated. 

The scientific method and discovery in the 19th century

Before this point, most of the theories were rooted in the fact that the scientific method was not well-developed and the lack of knowledge about microorganisms meant that people could not identify the true cause of the plague. Medical understanding was based largely on classical texts and empirical observations without a means to verify causation.

Yersin’s investigation led to the truth and further observations then linked the plague in origin to the rats and fleas of the city. Yersin arrived in Hong Kong in June 1894 during a severe outbreak. He set up a small laboratory in a straw hut provided by the French consulate because he was denied access to local facilities controlled by other scientists.

Yersin examined the bodies of plague victims and took samples from their swollen lymph nodes (buboes which is where the origin of bubonic plague is from). He used a microscope to observe the bacteria in fluid samples taken from buboes. He noticed small, rod-shaped bacteria with a characteristic bipolar staining pattern (resembling a safety pin), which was different from the bacteria other competing scientists had reported. He grew cultures of the bacteria on nutrient agar plates. His observations and experiments showed consistent presence of the same bacterium in multiple samples from different victims.

To further confirm his hypothesis, Yersin injected the cultured bacteria into guinea pigs and rats, which subsequently developed symptoms of the plague, demonstrating the pathogen’s role in causing the disease.

Rats & fleas: the ultimate vectors for the plague

After the identification of Yersinia pestis as the causative agent of the plague by Alexandre Yersin in 1894, the next critical step was to understand how the bacterium was transmitted. The discovery that fleas and rats were the primary vectors involved a combination of observational studies, field research and laboratory experiments

Observational studies

1. Researchers noted a correlation between plague outbreaks and increased rat mortality. Dead rats were often found before human cases of the plague appeared. 
2. The pattern of the disease's spread in port cities suggested that it might be linked to rodents and their movements, as rats were common on ships.

Field research

3. Investigations by Paul-Louis Simond 1898, a colleague of Yersin, were done in India. He observed the disease spread even when there was no contact between sick and healthy people. This indicated there may be another method of transmission, a possible intermediate vector

Simond conducted experiments that demonstrated that fleas could transmit the plague. He showed that fleas from infected rats could carry Yersinia pestis and transmit it to healthy rats. In his key experiment, he placed healthy rats in cages with infected fleas but without direct contact with infected rats and the healthy rats developed the plague.

Additional laboratory experiments also had researchers who isolated Yersinia pestis from fleas collected from rats, proving fleas were the ultimate carriers of the bacterium. It was found that fleas would regurgitate the bacteria when they bit a host if their digestive tracts were blocked by Yersinia pestis, thereby transmitting the plague bacterium to new hosts.

There was also strong epidemiological evidence such as the fact that many cases of the plague involved a characteristic flea bite on the legs and ankles. Epidemiological studies further supported this showing that controlling flea populations reduced plague transmission.

The plague today

According to local news from Oregon in the United States, there was a case earlier this year, in February 2024, where a man is believed to have gotten the plague from a pet cat. Although the plague is readily treatable with antibiotics, its symptomology can be confused with other infections. 

Bubonic plague symptoms include sudden high fever, chills, headaches, body aches and large swollen lymph nodes that leak pus. 

Additionally, while the bubonic plague is the most common form of the plague, there are other variants such as the septicemic plague that can cause blackened tissue and gangrene, as well as issues breathing and coughing up blood in the case of the pneumonic plague.

The plague is still fatal if not treated promptly. The plague can be cured with antibiotics such as Ciprofloxacin, levofloxacin and moxifloxacin.

Summary 

The determination that fleas and rats were the primary vectors for Yersinia pestis involved a combination of detailed field observations, experimental research, and epidemiological studies. The work of scientists like Paul-Louis Simond was pivotal in identifying the role of fleas in transmitting the plague from rats to humans. This understanding led to more effective public health measures, such as improving sanitation, controlling rodent populations, and targeting flea infestations, which have been critical in managing and preventing plague outbreaks.