It gets a little confusing, the way people tend to use the term “plague” willy-nilly without bothering to clarify. Allow me then to elucidate. A “plague” can be anything that attacks us as human beings in large numbers and/or with enough of a detrimental impact: any disease outbreak, for example, or a sudden explosion in the fly population, or Justin Beiber music. But THE plague, properly Plague with a capital P, refers to three primary strains of disease, and several less common strains, caused by the bacteria Yersinia pestis. The three big ones are Bubonic (aka the Black Death, the one that decimated Europe during the Middle Ages), Septicemic, and Pneumonic. Among the lesser known strains is Necrotic, which results in the reanimation of recently deceased human tissues. There, now, don’t you feel all educated and stuff?
Alright, I made up that last part. Plague doesn’t reanimate dead bodies. But a victim suffering through the latter stages of the disease would certainly have resembled one of the walking dead, and, if the infected had one of the rarer forms of plague called Meningeal Plague, which attacks brain matter, then they might well behave like a zombie, too. And the pestilence could be spread by a bite, so there’s another similarity. As far as metaphors go, zombies as stand-ins for the Plague is a pretty good one.