The Bubonic Plague is a deadly disease carried by fleas and is incredibly contagious, deadly, disgusting and also deadly, and by the time you start showing signs of it, it's pretty much too late. In the Middle Ages the phrase went, "You can have lunch with your friends, catch the plague, and then have dinner with your ancestors in paradise because you are totally dead".
The Black Death, so called because of the black spots that appear on infected persons' arms, first arrived in Europe on board an Italian merchant vessel in 1347. Over the next six years, the Plague swept through Europe faster than an outbreak of Zombieism, killing close to twenty-five million people -- over one-third of the entire population of Europe. People lived in the grip of fear for years, abandoning their towns and their families and quarantining the infected in an effort to rid themselves of this killer. However, it turns out they should have just stopped living with rats and maybe that would have solved their problems.
While the biggest outbreak of Plague was in the 14th century, there have been reports of it throughout history and up to the present day, as many wild rodents such as squirrels and rats in the Western portion of the United States are still known to be carriers for Bubonic Plague. The most recent documented death from Plague in America was in 2003.
Plague is badass because especially during the middle ages there was no way to fight it, and in six years it killed more people than war, famine, heart disease or any other epidemic in human history. It was an outbreak of near-biblical proportions. Plus, it's not like you could be so tough that you couldn't catch it and die. It's like, it doesn't matter how much of a badass you are, you'd still get your ass kicked by the Black Death.