The Black Death

This disease has come down through  history with a number of names, the Black Death, the Black Plague, the Bubonic Plague.  It has probably swept through the populated world at least three times, once in the 542-543 AD (The plague of Justinian) , in 1347-1353, and in 1855, spreading from Yunnan province in China and finding its way to India and even the US.    In the 14th century it killed about 30 to 60% of the population of Europe, not to mention parts of Asia, Africa, and the Middle East.  The 14th century pandemics were followed by periodic and more limited waves of plague 

 At the time there was very little understanding of what caused the disease, other than it was highly contagious, very likely to result in

death, and  its symptoms invariably included gangrene in the fingers and toes, where the skin turned black.  Another symptom was swelling in the groin, the armpits, and the neck known as buboes, which gave the disease its modern name, the bubonic plague. 

The result was the depopulation of certain parts of Europe, political and religious upheavals,  Geoffrey Chaucer's "The Canterbury Tales" and Boccacio's "The Decameron" were literary expressions of the ferment that resulted from all of world society being turned as it were, upside down.

Geoffrey Chaucer

The disease agent is Yersinia pestis, a gram negative coccobacillus and facultative anerobic bacterium.  What that means is that it has a round to rod like shape, it does not stain purple using the Gram stain  ( a basic method of classifying bacteria), and as a facultative anaerobe, it can take oxygen or leave it, depending on the circumstances.    Bacteria can be found just about anywhere and most of them in

Fluorescent Stained Y. pestis

most contexts are harmless, but not Yersinia pestis.  It started out as a disease of rodents and fleas and remains such a disease from the sewers of China to the prairie dog towns in the American great plains.   Rats and all small mammals are infested with fleas, the fleas are infected with Yersinia pestis, which grows in its gut, and when the flea regurgitates its bacterial load while feeding on the blood of the rat, thus giving the rat the plague.  The fleas

Flea engorged with Y. pestis

probably don't much like the infection either, but their health is of least concern.   The rats or other rodents   suffer some ill effects from the infection as well, but from long exposure to the disease, natural selection has seen to it that they live long enough to spread the disease. Occasionally since rats live in close proximity with humans, this led to humans being infected as well.

Diseases, when they jump species tend to be especially severe in the species that is new to the organism.  Thus it was that what may have made rats merely sick, made most humans die.  This combined with the Silk road, which allowed commerce to travel from the China to Europe, and with water routes, where rats lived and prospered in the holds of ships, going ashore along the ropes used to lash the boat to the pier, sharing their infected fleas with rats living in the port, and both sharing their disease with the sailors and port personnel who were bitten by the infected fleas.  In central Asia the flea-rodent reservoir was a variety of ground rodents as well as marmots.

Marmot

 

The plague is thought to have originated in the steppes of Central Asia along the silk road.  Nestorian graves in Kyrgyzstan in 1338 mention plague and this is thought to have been the time and the locus of the initial outbreak. This means that the plague spread to the far reaches of Europe within less than 15 years.   It spread west and arrived in the Crimea either by the silk road or by ship by 1346.  From there it spread rapidly to ports and islands in the Mediterranean in 1347, throughout southern Europe by 1348,  to England and Germany by 1349, to Scandinavia by 1350 and throughout Scandinavia and Russia by 1353. 

It is said to have reduced the population of the planet by around 1/5 by 1400.  It returned periodically from time to time after that.  It struck London in 1603 and again in 1665-6, Vienna in 1669,  Italy in 1629-31, Seville in 1647-52,  Marseille in 1720-22, Eastern Europe in 1738, and Russia in 1770-72.

When the first wave came through it reduced the population of Europe by about 50%, especially in the urban areas where people lived close together.  In the southern areas of France, in Italy and Spain it was estimated to be as much as 70-80% mortality. 

A real scientific understanding of the disease and its epidemiology had to wait for the late 1800s. 

At that time during an outbreak in China, Alexander Yersin (for whom the pathogen is named) identified the bacterium that caused the disease, it was named Yersinia pestis.  The epidemiology was worked out by Paul-Louis Symond in 1898, establishing the way in which fleas and rodents spread the disease.  Apparently the disease can be spread through aerosols, as in Pneumonic plague, or from flea bites as in septicemic plague or bubonic plague.  Analysis of trace DNA from persons who died in the earlier centuries established the identity of Y. pestis as the cause of the great pandemic of 1338 to 1353.

Chinese immigrants and trade with China   probably brought the plague to the US with an outbreak  centering on San Francisco in 1900-1904 and another wave of plague occurring in 1906.  That was the year when most of San Francisco was destroyed by an earthquake and then a fire.  Plague then passed into the local ground squirrel population and has since become endemic among wild rodents and the mammals that prey on them. 

In recent years it has decimated prairie dogs, a ground dwelling rodent common in the Western US.  Occasionally people become infected, getting the disease from their pets. In one case a wildlife biologist working in the Grand Canyon National Park   contracted it from a mountain lion on whom he was performing a necropsy, and before anyone suspected what was going on, he was dead.   Especially in the western US it is wise to take serious precautions when handling the corpses of wild animals or game. 

Except for cases such as these, the increased awareness of how disease spread and  improved sanitation of most cities in the last century has much reduced the possibility of such pandemics from occurring in the future.