Falling ill is not a form of failure or defeat. It doesn’t happen because our faith is weak. When the suffering of illness occurs amid our efforts for kosen-rufu, it is the working of devilish influences trying to obstruct our attainment of Buddhahood. As such, we mustn’t let illness intimidate us. The Daishonin teaches us how to bring forth courage to face illness and attain Buddhahood in this lifetime.

From The Wisdom for Creating Happiness and Peace, part 1, revised edition, p. 251