Evil has to do with anything that impairs one's happiness or welfare or that deprives one of good. Disease is evil; so are the various influences that tend to disrupt and demoralize human life. From the standpoint of one's experience, evil may appear to be very real. But from the standpoint of divine logic, evil is deception, not reality.
"God, Spirit, alone created all, and called it good. Therefore evil, being contrary to good, is unreal, and cannot be the product of God," writes Mrs. Eddy in "Science and Health with Key to the Scriptures" (p. 339). And she states farther on of the individual who makes that real which is unreal, "He is joining in a conspiracy against himself,—against his own awakening to the awful unreality by which he has been deceived."
The Biblical story of fallen man accords reality to evil. If we accept this story as fact, it would confirm man's sinful nature and tend to induce mankind's submission to evil. But if we see it as an allegory, it can teach us a valuable lesson; it can turn our thought to man's sinless nature as God's own likeness and enable us to overcome evil's false claims.