How is pneumonia described in the story?

In "The Last Leaf," pneumonia is personified in different ways. First, O'Henry refers to pneumonia as "a cold, unseen stranger." This stranger "stalked" about Greenwich Village and laid his "icy fingers" on his victims. The disease is also compared to a "ravager." O'Henry also says that pneumonia is not chivalrous because he attacks a little woman, meaning Johnsy. Pneumonia is also called "the red-fisted, short-breathed old duffer." 
These images make the reader call to mind a ruthless and revolting old man who wheezes, who is cold at heart, and who has fingers made of ice. By personifying the disease, O'Henry makes Johnsy's opponent seem real and makes the reader want to rally behind Johnsy in her fight against pneumonia. It is only hope and faith that will be able to help Johnsy defeat this enemy, and until she has something to believe in (which turns out to be the last leaf clinging to the ivy vine on the brick wall outside her window), she can't begin to defeat her opponent, pneumonia. 

In the story, pneumonia is personified as a bold "ravager" who stalks a community unawares and smites his victims "by scores." The author tells us that Mr. Pneumonia is a "red-fisted, short-breathed old duffer." 
Based on the author's description, Mr. Pneumonia can be compared to the Grim Reaper, which is the personification of death. In the story, pneumonia acts like the Grim Reaper. He is a mercurial killer in that his choice of victim seems arbitrary.
Mr. Pneumonia's latest victim is Johnsy, whom the author describes as a "mite of a little woman with blood thinned by California zephyrs." Interestingly, it is another "old duffer" who comes to Johnsy's rescue; to placate Mr. Pneumonia, Mr. Behrman (an aging artist who lives in Johnsy and Sue's building) offers himself up in Johnsy's place.
Johnsy, discouraged by the turn her life has taken, tells Sue that she will soon die when the last leaf falls off the vine outside her window. However, the leaf never falls off, even after a powerful storm. Encouraged by the leaf's will to live, Johnsy soon recovers. Later, she discovers that Mr. Behrman spent long hours outside painting the last leaf on the vine. The leaf was a masterpiece, so realistic that Johnsy was none the wiser. Mr. Behrman's sacrifice was an unselfish act of love.