The Black Death ravaged Europe between 1347 and 1353, killing millions. Plague outbreaks in Europe then continued until the 19th century.

One of the most commonly recited facts about plague in Europe was that it was spread by rats. In some parts of the world, the bacterium that causes plague, Yersinia pestis, maintains a long-term presence in wild rodents and their fleas (opens in new tab). This is called an animal “reservoir.”

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While plague begins in rodents, it sometimes spills over to humans. Europe may have once hosted animal reservoirs that sparked plague pandemics. But plague could have also been repeatedly reintroduced from Asia. Which of these scenarios was present remains a topic of scientific controversy.

Read more: Live Science