Hype aside, what do we know?
The Middle Ages called what was happening "The Great Pestilence."
The "Black Death" was a term coined in the 1600s to refer to the the grimness of the event.
The "Bubonic Plague" came into currency around 1885-1890 because of the swellings/bruises that resulted from infection. The Latin for bruise is "bubo, bubonis."
Yersinia pestis, isolated in the 1880s by bacteriologists Alexandre Yersin and Shibasaburo Kitasato (who were working independently; you can guess who "published first"), loves to breed in the digestive tract of the flea. Because its prolific duplication prevents the flea from being able to digest blood, the flea travels from host to host, biting furiously in an attempt to avoid starving. This causes bacteria to spill out and into the bloodstream of the mammal. Here's a pictorial view.
If you would like a more detailed graphic of how it works (and understand biochemistry more than I do), click here.
A dozen or so cases of Bubonic Plague are diagnosed and treated each year in the United States. The victims invariably (unless they exist solely for an episode of House) live in areas of poor sanitation.