There have been only three major outbreaks of Y. Pestis (each one of them followed by a century or so of periodical resurgences in the populations that had been affected): the first one after 542, the Justinianic Plague; then the Black Plague in the 14th century; and a Sino-Indian outbreak in the late 19th century. In Antiquity only, I know of at least three other epidemics that are often referred to as “Plagues.” The first one is the Plague of Athens, a disease that appeared during the Peloponnesian War, and was described in detail by Thucydides; it apparently swept across the Eastern Mediterranean, coming from Ethiopia or the Upper Nile. There is no scholarly consensus on its identification, but it was certainly not Y. Pestis; it may have been typhus. Then there are the famous Antonine Plague (2nd century) and the less famous Plague of Cyprian (3rd century), which owe their relative fame to the existence of good accounts of their effects (written, respectively, by Galen and Cyprian). Once again, medical descriptions are not precise enough for us to be certain of the nature of the disease, but the most likely suspect is smallpox. I don't know, however, if the word has been used for post-Justinianic epidemics (though common sense suggests that it was; people like to give impressive names to things, and “plague” certainly is a frightening label).