 The disease was spread to humans by rodents such as the black rat, which were bitten by infected fleas.Major outbreaks included the Plague of Justinian, c. 540 and the Black Death, c. 1350, both of which killed a sizeable fraction of the world's population.Fleas appear in human culture in such diverse forms as flea circuses, poems like John Donne's erotic "The Flea", works of music such as by Modest Mussorgsky, and a film by Charlie Chaplin.