Final answer:
Throughout different plague outbreaks, such as the Plague of Justinian and the Black Death, up to 50 percent of Western Europeans died, considerably reducing Europe's population and altering its societal dynamics.
Step-by-step explanation:
The fraction of Western Europeans who died from the plague varied across different outbreaks. During the Plague of Justinian, which occurred between 541 to 750 A.D., it is estimated that the population in Europe declined by 50 percent. This bubonic plague outbreak was one of the first major instances recorded. Later, the Black Death, another devastating outbreak in the 14th century, further reduced Europe's population by an estimated 25 to 50 percent. The effects of these epidemics were profound, causing long-term population decline and impacting European society on multiple levels.