The Bubonic Plague (1347-1700s)
Bubonic plague is an acute infectious disease caused by the bacterium Yersinia pestis. The bacteria live in the intestines of fleas and are transmitted to rats by flea bites. The rats, therefore, serve as a natural reservoir for the disease, and fleas are the vectors. Occasionally, an infected flea would jump to a human and introduce the bacteria when a blood meal was taken. The bacteria would then spread to the regional lymph nodes and multiply, causing dark, tender, swollen nodules (buboes), as shown below in a boy a walnut-sized swelling in the inner aspect of his upper thigh. As the infection spread, the victim would experience headache, high fever, delirium, and finally death in about 60% of cases.
Starting in 1347, Europe experienced multiple waves of bubonic plague epidemics that lasted until the late 1700s. It is believed that the bubonic plague originated in Asia and traveled along trade routes into the Black Sea and then into the Mediterranean Sea. From there, it swept through Sicily and Italy and then up through France and the northern Europeancountries all the way up into Scandinavia. There were many subsequent waves of plague that swept through Europe until the late 1700s.
Cause of the Plague and Strategies for Prevention The cause of the plague was not known, but there were many theories. The most popular explanation was that it was caused by "miasmas," invisible vapors that emanated from swamps or cesspools and floated around in the air, where they could be inhaled. Others thought it was spread by person to person contact, or perhaps by too much sun exposure, or by intentional poisoning. The miasma theory was the most popular, however. One of the popes kept large fires burning at both ends of the room he worked in order to counteract the miasmas.
There were also crude medicines that were concocted to prevent or cure the bubonic plague; one of them was known as theriac. Of course, smoke and aromatic herbs and theriac were ineffective, because the plague was primarily spread by flea bites (although sometimes victims developed a plague pneumonia that caused them to cough up a bloody, plague-filled aerosol that could be transmitted to others by inhalation; this was the 'pneumonic' form of the plague).
While most believed that plague was caused by miasmas, the primary mode of transmission was actually via flea bites, and, in a sense, the real causes were increased population density and failure to dispose of garbage. Accumulations of garbage attracted rats and enabled the rat population to explode. Rats had harbored fleas and Yersinia pestis for many years without major difficulty, and plague epidemics in humans didn't occur until human behaviors created environments that brought people into proximity with rats, fleas, and Yersina pestis. These were the real causes of the plague epidemics.
At first glance one might blame the lack of understanding about transmission and the ineffective preventive measures on the primitive level of scientific understanding. However, the inability to identify the cause and the inability to identify effective control measures was not due to a lack of sophisticated technology. Instead, it was primarily due to the fact that humans had not yet developed a structured way to think about the determinants of disease. There were certainly theories of how the plague spread and these led to preventive strategies, but none of the theories or preventive strategies or treatments were ever tested by collecting observations in groups of people.
The idea of studying groups of people to test associations between "risk factors" and disease outcomes had not yet evolved.
Key Concept: The lack of a systematic way of testing possible associations between exposures and outcomes ("risk factors" and disease) was the major factor that prevented advances in understanding the causes of disease and the development of effective strategies to prevent or treat disease.