Last night I awoke in the middle of the night wondering about the etymology of the word "plague." As one does. The same word means both "a widespread illness" and "to be annoyed." That seemed a bit odd to my 2AM, sleep-addled self, so I resolved to look it up first thing in the morning.
The earliest meaning of the word "plague" was a broad one. It included any calamity or affliction, including wounds, blows, and misfortune. According to the Online Etymology Dictionary it comes from the Latin word plaga meaning "to strike or hit," which was itself stolen from the ancient Greek term plaga meaning "blow." It shares a common origin with an Old Frisien term meaning "to curse," and it shares a common root with a Latin term meaning "to lament by beating the breast."
So there is this connotation of concussion and force, even in the earliest versions of the word. We think of "curse" as a somewhat passive activity, something that a witch does by brooding over a bit of your hair and a bubbling pot of potions. But cursing used to be a much more physical, active activity. Witness the use of the forked fingers to ward off the evil eye in public, a tradition which is still alive and well in many Eastern European cultures.
Plague came to be associated with disease and annoyance in the 1500s. Its association with pestilence came first, and annoyance was recorded in the 1590s. Which is kind of funny if you think about it, since pestilence was anything but a minor inconvenience in the 16th century!
It wasn't until the 1600s that "plague" became even more specifically tied to bubonic plague. Ever since then, most of the time when you hear the term "the plague," people are specifically of bubonic plague. Interesting that this happened so late, considering that it is 200 years after the Black Death wiped out a third of Europe. I suppose it happened with the scientific enlightenment and mania for taxonomy that hit Europe (a plague of its own, if a happy one) during that time.
As a side note, "plague" is also the collective noun for grackles, as in, "a plague of grackles." I find this funny because grackles really are a plague in many areas. They are one of the few animal species (along with deer and mice) which thrive with the spread of the human population, and their aggressive habits often drive out more mild-mannered local bird species.