On that day it is the duty of Amis to fight in a trial by battle.

But he is sick, and can not fight; then to save his honour his friend Amile puts on the armour and helmet of Amis, and so pretending to be Amis, goes to the meeting place, and wins the fight gloriously.

But this was an act of untruthfulness; he had gone into battle under a false name, and to do anything false even for a good motive is bad.

So heaven punishes him by afflicting him with the horrible disease of leprosy.
The conditions of leprosy in the Middle Ages were of a peculiar kind.

The disease seems to have been introduced into Europe from Asia--perhaps by the Crusaders.