How can the statement "A diseased mind is even more harmful that the disease itself" be justified with reference to O. Henry's "The Last Leaf"?

The idea that a "diseased mind" can be more dangerous than the actual disease is exemplified in Johnsy's defeatist attitude that the doctor knows will prevent her from recovering from pneumonia.

In the exposition of "The Last Leaf," Johnsy, Sue's friend, has succumbed to pneumonia because she is from California and not acclimated to New York winters. After the busy doctor makes his house call and examines Johnsy, he tells Sue the young woman has

one chance in—let us say, ten. . . And that chance is for her to want to live. . . Your little lady has made up her mind that she's not going to get well.

The doctor then asks Sue if Johnsy has a significant other she loves, but Sue replies, "there is nothing of the kind." The doctor says that when people start to think of dying, he subtracts "50 per cent." He promises to do all he can with medicine, and departs. Sue goes into the room where Johnsy lies and tries to finish her sketching. Then, she hears Johnsy counting. Johnsy tells Sue that she has been counting the ivy leaves on the brick wall outside her window as they fall, and there are only five left. "When the last one goes, I must go, too."

Sue tries to cajole Johnsy out of such thoughts, but Johnsy insists that she must die when the last leaf falls from the vine. So, Sue tells her friend to try to sleep while she goes downstairs to get Mr. Behrman to model for her.

Once there, Sue informs Mr. Behrman of Johnsy's condition and her determination to die when the last leaf falls from the vine. The little curmudgeon becomes irate when he hears what he calls "foolishness," asking Sue,

Vy do you allow dot silly pusiness to come in der brain of her? Ach, dot poor leetle Miss Yonsy.

Sue explains that Johnsy is weak and the fever leaves her mind weak and "full of strange fancies." Despite his grumbling, Mr. Behrman accompanies Sue upstairs. There they see Johnsy sleeping and observe the ivy vine with fear because more leaves have fallen from it.

The next day, Johnsy asks Sue to roll up the shade. Fearfully, Sue obeys; against the brick wall, there is yet one leaf on the vine. Sue begs Johnsy to think of her and live, but Johnsy is "merciless." The next day, however, the ivy leaf somehow has seemed to remain on the vine.

"I've been a bad girl, Sudie," concludes Johnsy, "Something has made that last leaf stay there to show me how wicked I was. It is a sin to want to die. You may bring me a little broth now and some port in it, and. . . I will sit up and watch you cook."

After Sue summons the doctor again, he gives Johnsy "even chances" to become well. He also sadly informs Sue that Mr. Behrman has died of pneumonia after being discovered by the janitor in a state of dampness. Later, Sue learns Behrman went out after posing for her and painted a leaf on the bricks so Johnsy would think it survived the storm and she should get well. Behrman's "masterpiece" gave Johnsy the will to live so that her mind could then triumph over her body's illness.