Crusoe is extremely lucky to have found himself on an island where there is shelter and an abundant food supply. However, things are not all plain sailing. After about a week of heavy rainfall, Crusoe is struck down with a particularly nasty bout of fever complete with a splitting headache.
In his feverish state, Crusoe starts having scary hallucinations in which he's told by a man with a weapon in his hand that his suffering has not yet brought him to his repentance. The implication here is that Crusoe has turned away from God and that his suffering is meant to bring him back.
After he wakes from his vision, it seems that Crusoe has learned his lesson, and he offers up a genuine prayer to God for the very first time. But, as the man in his fever dream told him, he hasn't suffered enough, and so Crusoe is assailed by thoughts of extreme self-pity.
Eventually, Crusoe falls asleep for over twenty-four hours. On waking up, he finds that he has completely recovered and gives thanks to God for his truly remarkable recovery. From now on, he will begin an in-depth study of the New Testament, a sure sign of how much he has been changed by his terrible illness.