How does Old Behrman catch pneumonia in "The Last Leaf" by O. Henri?

O. Henry introduces the subject of pneumonia early in "The Last Leaf."

That was in May. In November a cold, unseen stranger, whom the doctors called Pneumonia, stalked about the colony, touching one here and there with his icy fingers. Over on the east side this ravager strode boldly, smiting his victims by scores, but his feet trod slowly through the maze of the narrow and moss-grown “places.”

In O. Henry's day little was known about the...

O. Henry introduces the subject of pneumonia early in "The Last Leaf."

That was in May. In November a cold, unseen stranger, whom the doctors called Pneumonia, stalked about the colony, touching one here and there with his icy fingers. Over on the east side this ravager strode boldly, smiting his victims by scores, but his feet trod slowly through the maze of the narrow and moss-grown “places.”

In O. Henry's day little was known about the disease, and doctors had no antibiotics to treat it. According to the Mayo Clinic:

A variety of organisms, including bacteria, viruses and fungi, can cause pneumonia.

Most of the people in New York must have been exposed to organisms that could cause pneumonia, but it was mainly those who were susceptible for one reason or another who caught it and died. Old Behrman is a good candidate for pneumonia even before he goes out on a stormy night to paint a fake leaf on a brick wall. He is old. His heavy drinking has  affected his health. He doesn't take good care of himself. He doesn't dress warmly. O. Henry intentionally shows how Behrman dresses inappropriately for the weather.

A persistent, cold rain was falling, mingled with snow. Behrman, in his old blue shirt, took his seat as the hermit miner on an upturned kettle for a rock.

Even indoors in those cheap living quarters in Greenwich Village, it must be very cold inside. When Behrman goes outside in the dead of night to paint that single ivy leaf, he is probably still wearing only "his old blue shirt." It is no wonder that he succumbs to pneumonia, since it is going around and since he exposes himself heedlessly to cold, wind, and rain.