Unsanitary city conditions, not just rats, caused the Black Death to spread with such devastating speed.

The Black Death, caused by the bacterium Yersinia pestis, ravaged Europe between 1347 and 1351. While the traditional narrative blames the black rat (Rattus rattus), modern research from the University of Oslo suggests a different primary culprit. Researchers used mathematical models to track the spread of the disease and found that human ectoparasites, such as body lice and human fleas, better explain the infection patterns observed in historical records.Medieval cities were incredibly dense, often housing thousands of people within cramped stone walls. Cities like London and Paris lacked formal waste management, leading to streets filled with household refuse and human waste. This environment was a breeding ground for parasites. Because people lived in close quarters and rarely changed their wool clothing, lice could easily migrate from person to person, carrying the bacteria with them.The speed of the pandemic was unprecedented, moving roughly 1.5 kilometers per day across the continent. If rats had been the sole carriers, the spread would have been much slower due to the limited range of rodent colonies. By 1351, the plague had claimed an estimated 25 million lives in Europe alone. This high mortality rate was exacerbated by the Great Famine of 1315 to 1317, which had left the population malnourished and immunocompromised decades earlier.