The 5,000-year journey of a bacterium that reshaped human history

Health · Published 18 October 2026 Swipe more like this →

In short: Plague is one of humanity's oldest enemies, but its history is more than just a series of tragedies. From Neolithic Sweden to the Silk Road, the bacterium Yersinia pestis has evolved alongside us, adapting its very genetics to hitchhike on fleas and travel across continents.

The presence of Yersinia pestis is not a modern phenomenon. Genomic traces found in the teeth of Neolithic farmers in Sweden date back roughly 4,900 years. Further analysis of Bronze Age human teeth reveals the bacterium was already established across Asia and Europe between 3000 and 800 BCE. Some researchers even suggest the plague may have contributed to the decline of Neolithic societies, evidenced by a tomb in Sweden containing 79 corpses buried in a short period.

The disease's path into the modern era was marked by three great pandemics. The first, beginning in 542 CE, struck Constantinople and killed tens of thousands, spreading via maritime trade routes. The second was the devastating Black Death of the 14th century, which killed an estimated one-fourth to one-third of Europe's population—roughly 25 million people. The third pandemic began in the 1850s in Yunnan province, China, eventually reaching global ports like Hong Kong and San Francisco by the turn of the 20th century, resulting in over 10 million deaths.

The evolution of the plague is a masterclass in biological adaptation. A key mechanism is the Yersinia murine toxin (ymt), which allows the bacteria to infect fleas. Crucially, early ancestral versions of Y. pestis lacked this gene; it was only detected in a sample calibrated to 951 BCE. This shift enabled the transition to flea-borne transmission. While medieval observers attributed the disease to 'poisonous miasmas' or divine wrath, modern science eventually isolated the organism in 1894, leading to the germ theory of disease and a deeper understanding of how pathogens move through trade and climate fluctuations.

Today, the plague remains a persistent zoonotic threat. While it subsided in western Europe after the 17th century—likely due to improved sanitation and housing changes that excluded rats—it continues to appear in regions like Turkey, North Africa, and India. Modern surveillance focuses on identifying ecological and social determinants to prevent the next major outbreak.

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