Who stalks about the colony in “The Last Leaf”?

In O. Henry's short story "The Last Leaf," "a cold, unseen stranger, whom the doctors called Pneumonia, stalked about the colony, touching one here and there with his icy fingers."
There is no literal person who is stalking the colony. Instead, this is O. Henry's way of expressing that a pneumonia epidemic has struck the colony. O. Henry uses personification and anthropomorphism to express how unwelcome and feared the pneumonia epidemic is. Like a strange and malevolent person who sneaks through the houses, pneumonia strikes down several people in the colony. It sneaks up on them and damages their lives, and there's no way for them to tell who might be hit next. O. Henry comments that this pneumonia is not a polite or chivalric presence, because it picks on the weak who have no chance of fighting him off. Describing pneumonia as a cunning adversary instead of just a disease increases interest in the central conflict of the story and elicits more sympathy for the main characters.