The Black Death is the name given to three kinds of plague that were related but slightly different. These plagues created a pandemic that killed over 25% of the population of Europe in the late 1340s and early 1350s.
The majority of plague cases were of bubonic plague. This is spread by fleas carrying a bacterium, Yersinia pestis. The fleas lived mainly on rats and spreaed from them to people. If the plague bacteria spread to the lungs, it was called pneumonic plague. This could be transmitted from human to human by coughing. There was also septicaemic plague, which was carried by insects but was very rare.