The Black Death that swept across Europe in the 14th century did indeed wreak absolute havoc on the population. Of course, this was the part of the world that was the most populated at the time, so it also meant that a huge percentage of the entire global population were affected by this disease.
But then, how big a problem was it?
Well, even though we do not have exact figures at hand, it is estimated that as much as two-thirds of the population of Europe at the time died as a direct result of the disease. Even if the idea of 66% of people seems to be high, the vast majority of the studies that have been carried out into the plague do tend to put the average at more than 50%, which is still pretty impressive—in a horrible way.
Also, if you want to put this percentage into real numbers, the estimates vary even more widely due to the way in which the population was spread across the continent. However, even with that, the figure can be as high as 200 million people or as low as 75 million, but even then, you are looking at a large number of people in comparison to population figures at the time.
This was not even the only time that the plague swept into Europe, but it was certainly the point where it really did take its toll, and Europe as a whole was viewed as being quite lucky to even recover from it in the way that it did.