Piers Plowman is strongly influenced by the plague, or the Black Death, to give it its more famous name, which is demonstrated in its setting and characters. The Black Death devastated the English countryside every bit as much as the rest of Europe. Entire villages were decimated; towns became ghost towns full of empty, decaying houses.
The plague had a particularly disastrous effect upon the rural economy. With fewer hands available to help bring in the harvest large tracts of agricultural land fell into disuse, turning to wasteland and scrub. This had a significant knock-on effect on the structure of rural society. The old feudal system was quickly undermined as the huge labor shortage meant that those working the land could command higher wages—and as they were no longer tied to the land they could, for the first time in their lives, move elsewhere to make a living. Many of these laborers moved to towns, which led to increased urbanization and the growth of the urban economy.
This, then, is the background against which Piers Plowman is set. The countryside has been turned upside-down by the plague; the old certainties have vanished. In its stead we have a chaotic society in which all classes of people have departed from the traditional path of Christianity. Langland uses the plague and its terrible aftermath as an opportunity to moralize, to try and bring both noblemen and common folk alike back to the godliness which they were supposed to have displayed prior to the Black Death.
The social decay caused by the plague is used by Langland as a metaphor for the spiritual descent of rural society as a whole. He clearly resents the changes which have overtaken the countryside, and in particular the increased urbanization to which they've led. At the same time, he sees this as an opportunity to transform the new society, one in which the growing poverty of the towns and the corruption of Church and government are challenged on the basis of the Word of Christ.
For Langland, plague and sin are closely related. Plague is a sign of the apocalypse, a sign that mankind is sinful and must be punished. In that sense, the plague could be interpreted as an expression of divine wrath. More than that, it acts as a purgative to society, cleansing the rural social system of lazy peasants who refuse to work. For only through
. . . pryde and pestilence shal moche peple feche.
Pestilence acts as a social and moral corrective, removing the stain of sin from society. To that end, it's necessary that sinners must die in order for us to build a godly Christian kingdom. For only then
. . . shal deth withdrawe and derth be iustice.
Divine justice can only be etsablished once the pestilence has desecended upon the sinners and carried them off to hell. Though Langland's Medieval worldview seems both strange and offensive today, there can be little doubt that his attitudes were widely shared at the time. A deeply religious society had been shaken to the core by a hideous disease it never fully understood. The only way such a cataclysmic event could reasonably be explained was as an act of God, sent by Him to remind us to heed and remedy our sinful ways.