"Be thankful to Allah. He has made this illness a thing to atone your sins because a disease in itself has nothing to bring reward to anyone, it merely expiates one's sins and so far as reward is concerned, one has to earn it with his good words and good deeds. The Almighty Lord grants Paradise to his creatures on account of their piety and noble thoughts."
Mola Ali (a.s) Ibn e Abu Talib (a.s)
Ref: Nahjul Balagha (Saying # 042)