In the middle of the 14th century, there was an even steeper decline in population across Europe... and Asia. This decline was so steep that it shows up on graphs of the human population over more than 10,000 years.

All Human Population.png

Based on a graph created by Waldir

This dip was due the spread of a deadly plague that wiped out between 30% and 50% of the European population died within a few years. This disease may also have killed as many as 25 million people in China, before crossing Central Asia. There is debate over whether it reached sub-Saharan Africa.

The exact impacts varied locally. Later, more localized outbreaks of diseases took place in the 1370s, 1390s, 1410s, 1430s, and beyond, causing further population loss.

Sources from all these regions mentioned similar visible symptoms in the victims of this disease:

Swollen and blackened/purplish lymph nodes in the armpits and groin
Bleeding into the skin
Fever
Weakness
Sudden death
Probably caused by the bacterium Y. pestis,
Combined with other infections?

Read This Read the account of the plague from Nicephorus Gregoras's Roman History (in Rosenwein, Reading the Middle Ages, no. 8.1)

You can compare this with an account of the plague from the History of News of Padua and Lombardy from 1256 to 1364.docx and account of the plague from the chronicle of Neuberg Abbey.docx (located in what is now Austria). If you can't see/ read these, please let me know.

Note: it is normally difficult to diagnose illnesses from historical sources, since they might not include all the symptoms or just some of the most dramatic ones. However, the similarities in all these written sources, along with archaeological evidence, suggest a culprit for this disease: Yersinia pestis.

Sources written by people living through this plague indicate their despair and the huge scale of the mortality. Take this excerpt from a letter written by the Italian Renaissance poet Petrarch to his friend Louis Heyligen:

"Scarcely a year and a half has passed since I returned to Italy and left you weeping on the banks of the Sorgues… Where are our dear friends now?... There was a crowd of us, now we are almost alone. We should make new friends– but how, when the human race is almost wiped out; and why, when it looks to me as if the end of the world is at hand?"

~ Letter written in May or June 1348, translated by R. Horrox, The Black Death (Manchester: Manchester University Press, 1994), pp. 248-49