Plague endemic in man since early Bronze Age, but has not always been flea-borne
This new research indicates that human populations have been infected by the plague for more than twice as long as previously thought; first occurring during the early Bronze Age. Furthermore, it has been discovered that the early forms of plague would have been spread predominantly through human-to-human contact.
The early forms of plague lacked the genes needed for it to survive in fleas and to enable it to inflict the infamous swollen buboes of the Black Death. Genetic mutations in the plague-causing bacteria (Yersinia pestis) that gave rise to the flea-transmitted bubonic form of plague arose around 1-2 thousand years BC. It was this bubonic plague that killed half the population of Europe in the 14th century.