Britannica Sources used in this answer

Why might plague have entered Europe in repeated waves from Central Asia?

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Medieval illustration of Brotherhood of the Cross flagellants in robes and pointed hats walking in procession, some carrying a crucifix and banners, during the Black Death. — Black Death Flagellants in the Netherlands scourging themselves in atonement, believing that the Black Death is a punishment from God for their sins, 1349.

Modern research suggests that the plague entered Europe in repeated waves from Central Asia primarily due to climate fluctuations. These environmental changes impacted rodent populations in Asia that were infested with plague-carrying fleas.12

As these rodent populations were affected, the disease was reimported into Europe multiple times via established trade routes, specifically those comprising the Silk Road. These routes, which saw a revival under the Mongols during the 13th and 14th centuries, facilitated the movement of the plague bacterium westward from Asia into European port cities.1234