The black death is caused by a gram-negative coccobacillus called Yersinia pestis, which actually has a few family members that can be fairly nasty also, such as Yersinia enterocolitica and Yersinia pseudotuberculosis. Like a large number of highly infectious diseases plague is an example of a zoonosis or a disease that is spread from animals to humans, in this case originating from fleas that live on rats. When the rat population becomes low, or the animals are predated on by the likes of cats, the fleas are forced to vacate home and find a new host. Very often, this host happens to be a nearby human and the bacteria are transmitted into the unfortunate person through the fleas bite.