Why does Allah create physical and psychological diseases, and make some of them afflict children through heredity, such as cancer, or cause psychological suffering, such as the case of obsessive-compulsive disorder, which robs one of happiness?

The origin of divine decree (Qadar (definition)) is a secret of Allah. Delving deeply into it is a cause for abandonment (khidhlan). Allah Almighty has concealed this knowledge from His creation and forbidden them from seeking its full understanding, as He states: "He is not questioned concerning what He does, but they will be questioned." One must be certain of Allah Almighty's wisdom in His decree and destiny, and that all His actions encompass perfect wisdom, even if minds are incapable of grasping it. Belief in the unseen is the foundation of religion. Everything Allah has created holds a benefit and wisdom in its creation, even if some of it contains a partial, relative evil, for "evil is not attributable to You." Allah dislikes something, yet this does not negate His willing it for the sake of something else, and for it to be a means to something more beloved to Him.