The precise origins of the Black Death are unlikely to ever be known, but we do know how it spread. Animals, in particular rats, carried fleas infected with the plague bacteria when they climbed aboard ships sailing around the world. The fleas then jumped onto humans, bit them, and infected them with the Black Death. In the 1340s, the trading routes that were bringing Europe new wealth and opportunities brought it something more sinister. With no knowledge of what was causing this devastating disease, people continued to move and trade, spreading the plague further and further afield.