Causes
Untill the late nineteenth century that the cause of the black death was found out. It was from a germ carried by oriental fleas that were carried over from black rats. The germ, or bacteria, was called Yersinia Pestis (Y Pestis). When the rats died, they moved on to humans , when the fleas bit the humans, the bacteria went into their blood stream, and in turn created the black death. Towns in the middle ages were crowded and dirty, so the plague was easily spread throughout the population. Although most people had fleas anyway, so they didn't notice.