A diseased mind is the one that stores diseased thoughts which harm the functioning of body.In the lesson Last Leaf by O.Henry Johnsy who was suffering from pneumonia , gave up on surviving and started comparing her with an old ivy tree which was on the verge of drying up.she thought that when the Last leaf would fall she would also plan her journey to heaven.Johnsy’s debilitating effect of the disease had not only weakened Johnsy’s body but also her mind.She fancied herself to be as fragile, delicate and sensitive as a decaying leaf.
But in the end she, on observing the last leaf standing still - which was painted by the land lord - she changed her mind to live thus the disease itself was cured.
Meaning of diseased mind is more dangerous than diseased body