When people use the term "Black Death" today, they are usually referring to the initial, transcontinental devastation of the 1340s that arrived in Europe in 1347. This wave of "pestilence," the most common term in medieval sources, blitzed over Europe in the next few years before evaporating--temporarily.

Over the next centuries, Europe was visited by many waves of pestilence that they recognized as the same disease. While these could be locally catastrophic, none of them ever approached the international, intercontinental cataclysm of the first outbreak.

Scholars have set forth a lot of plausible reasons for the diminishing severity. Improved coping mechanisms (quarantine, sanitation) lead the list. One factor that is often posed to explain the severity of the primary Black Death in western Europe is weakened adult immunity owing to having grown up during the Great Famine of 1315-21. Changing disease pathology/microorganism biology is another one that gets mentioned quite often, especially because the Y. pestis of the Middle Ages (which archaeology tells us was the plague bug) doesn't seem to be quite the same as today's Y. pestis.