The COVID-19 pandemic  continues to wreak  havoc on the world while changing ways to safeguard our health. To date, there have been more than 5.3 million cases worldwide and the death toll continues to rise.  Morbidity and mortality with COVID however pales in comparison to the  bubonic plague which occurred in the mid-14th century.  

The plague killed as many as 50 million people, although that number many range from 75 to 200 million. They all occurred within four years as the bubonic plague marched across Europe, Scandinavia, Northern Africa, the Middle East and Central Asia starting from the late 1340s. Anyone affected had a less than 50% chance surviving, and thus the disease was described as the Black Death. 

The term “Black Death” was appropriate for the sweeping rampages of the plague, and characterized by the black, necrotic tissue that often accompanied the disease in humans. Though a common term now, the Black Death as a reference to the plague of 1348-1352 and its following waves was not used until at least 100 years later.


Between 30 to 60% of the population of Europe at the time was devastated, and estimates suggest that the Black Death reduced the world’s total population from 450 million people to approximately 350 million. These numbers are questionable because accurate records were not kept at that time due to primitive administration and to the sheer enormity of deaths occurring on a daily basis.


The Black Death is caused by a bacterium called Yersinia pestis, named after Alexandre Yersin, the scientist and bacteriologist who discovered it in the 19th century. These bacteria are commonly present in fleas which are carried by rodents found primarily in the Orient.

The first and perhaps most commonly known type of plague is bubonic plague, so named because of the distinctive buboes that appear. Buboes are swollen lymph nodes infected with the Y. pestis bacteria.

Bubonic plague is usually transmitted by fleas carrying the bacteria or from coming into contact with bodily fluids from a deceased plague-infected animal. Within a week of contraction, the victim begins to suffer flu-like symptoms, all of which are the same as are described in medieval accounts: extraordinarily high fever, vomiting and headaches, as well as the possibility of the presence of pus-filled tumors.

Today, bubonic plague still has a death rate of up to 75%. Without treatment, 80% of contemporary victims die within a week of contracting the disease. Some antibiotics work, although vaccines have not proven to work effectively to prevent the disease. Bubonic plague still occurs with some frequency in parts of Africa.

The second type is pneumonic plague, and as the name suggests this affects the respiratory system. Its mortality rate is higher than bubonic plague at upwards of 90%. Symptoms are coughing blood and high fever. At the end of the life of a person suffering from pneumonic plague, coughing will induce free-flowing blood expelled from the lungs.

The third, least common, and arguably the deadliest type of plague is septicemic plague, which is an infection of the blood. It is the rarest of all plagues because it requires actual infection to take place in the blood. In most cases, bubonic plague or pneumonic plague stays either in the lymph or respiratory systems respectively.


The modern bubonic plague is almost never passed from one human to another. Even in its pneumonic form, it is a disease of rodents (i.e. black rats).  Almost invariably it requires a human being in contact with the right kind of flea from a vulnerable rodent, which is contact with a flea from a resistant rodent. Thus, two fleas and two rodents are needed to infect a human being. While the black rat (Rattus rattus) may be the prime vector, other rodents may be involved.

Incubation Time

The bubonic plague incubation period is two to four days before the onset of symptoms, with an outside range of one to six days.  Because of this short period, quarantine has no value.  Strangely, 14th century witnesses said that, while the disease seemed to kill within a few days of the appearance of symptoms — even in some cases without outward symptoms — a person could travel a long distance from an infected place without sickening, only to fall ill after being received into a new place.  As a result, European cities experimented with periods of isolation, and settled on a quarantine of 40 days.


In the decades leading up to recent times, it had been long been accepted by most scholars that the source of the Black Death pandemic had its origins somewhere in China — probably the Hubei province — where a climatic event drove rat colonies into contact with human populations sometime in the 1330s. After that the plague moved westward at a frightening pace, transmitted by caravans traveling the Silk Road. 

Final Thoughts

Some historians conflict with the association of Y. pestis as the cause of the 14th century Black Death. They point to the speed with which the disease travelled,  and killed; its symptoms,  its extraordinary morbidity and mortality rates; the shortage of evidence for rats and fleas; and the seasons and climates in which the disease thrived.

Max Sherman is a medical writer and pharmacist retired from the medical device industry.  His new book “Science Snippets” is available from Amazon and other book sellers. It contains a number of previously published columns.  He can be reached by email at