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how did the black death spread
We usually blame rats. But the killer was a bacterium named Yersinia pestis, and the rats were only part of how it travelled.
Its usual carrier was the flea. A flea that fed on an infected animal could pass the bacteria to a person with one bite.
A bite gave swollen glands called buboes: bubonic plague. If it reached the lungs, coughs could pass it on directly: pneumonic plague.
Which fleas did the work? The old story says rat fleas. A 2018 study found human fleas and lice fit the death records better, though others dispute it.
Then came the ships. The plague reached Sicily in October 1347, Marseille in early 1348, England by June, and Moscow by 1353, following the trade routes.
So the whole picture: a bacterium, carried by fleas and coughs, travelling the trade routes. Which creatures mattered most is still argued.
The Black Death travelled on tiny fleas, on coughs, and on ships, and each link made it hard to stop.