What is the ending of the last leaf?

In that moment joan regonized that mr.behrman was her guardian angel. He gives her the opportunity to think positive about her own life and that she doesn´t believes when the last leaf falls from the vine outside the window ,she will die.With the secretly paint a leaf on the wall -a leaf that would never fall, he inspiring Joan to fight mentally her illness. He gives his healthiness to save this from Joan. When I considering that,it was cold and wet from painting in the icy rain I realize that he also caught pneumonia and died.

So the surprise ending was the final realization that the last leaf was not real but a painting seemed to have a magical healing power to renewed Joans will to love and to defeat her pneumonia.