The bubonic plague kills you all kinds of ways. One of the more fascinating ways it kills you is engaging in a protracted war with your cells over the mineral rights to your body.
When virulent bacteria enter a host alone, they have almost no resources, and they use whatever resources are on hand. One of the resources critical to their life and reproduction is iron, but iron can be so tough to get that some bacteria skip it altogether and make do with magnesium. But when you kill so many people that you get the name "the plague," you don't settle for second best.
And so, when the bacteria responsible for the Yersinia pestis moves in, it does its level best to rip the iron right out of its hosts' cells. It does this the way all iron-containing bacteria mine for iron in an animal's body —it releases a little compound known as a siderophore, the strongest iron-binding agents out there. They tear iron out of the environment — some bacteria can be used to clear the rust from soil with their siderophores — and hold it until the bacteria comes along and consumes the iron, siderophore and all. When the bacteria gets enough iron, iron-dependent gene repressors keep it from expressing the genes that make siderophores and the cycle shuts down.
That shutdown is not likely to happen when the bacteria is in the human body. Humans are, relatively speaking, an iron-poor environment, and so the bacteria goes into full mining mode. The bubonic plague bacteria has a devilishly efficient siderophore called yersiniabactin. The human body has an iron-binding protein called lactoferrin, but it's no match for the plague. The yersiniabactin can rip the iron right off of it. Since stealing the iron back from the bacteria's henchman isn't an option, the body tries to at least keep the bacteria itself from getting the iron. It does so with its own protein, siderocalin, which eats the yersiniabactin, which ate the iron. It's a poor-old-lady-who-swallowed-a-fly situation, and someone definitely dies at the end of it.
Much of the time it's the person, if they are left untreated. This efficient iron collection system is one of the reasons why bubonic plague was so deadly. If deprived of iron, Yersinia pestis is in trouble. Researchers found that bacteria which had their siderophores repressed were far less virulent. Whoever controls the iron gets to live.