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Fleas and rats. The Black death or the Black plague spread through from Central Asia om the 1330s. The disease was brought up to Europe from Sicily by the Genoese of Kaffa in 1347 because when Kaffa was under attack by the Tartars, the plague killed most of them and the small handful of Tartars left panicked and started throwing the dead bodies of their comrades over the walls of the city hoping that the smell would kill them.
About 1/3 of Europe died over the course of 2 years. Some towns and villages became completely deserted due to plague victims and people fleeing the plague. When the plague finally burnt out, there was a much higher demand for labor than there was supply. This meant greater wages and rights for the peasants who would eventually do this work.