A disease can be cured by medicines and by a doctor's treatment, but a diseased mind has no cure with medicines or any such science. a disease, to b cured, needs a healthy and a strong mind and a will power to live one's life. even if one doesn't have any disease, once one's mind is diseased and it fails to be positive, the internal damage is often fatal.
in the story 'last leaf', Johnsy has got a weak mind that reduces the power of the doctor's prescribed medicines and therapies by 50 percent, as said by the doctor. we know that the curative power of the medicines along with Sue's treatment could be well effective on Johnsy much earlier if she did not possess the disease in her mind that she would certainly die.
so, it can be very well inferred that a diseased mind is more harmful than the disease itself.