It is difficult to read any historical account of smallpox without encountering the word filth. In the 19th century, smallpox was widely considered a disease of filth, which meant that it was largely understood to be a disease of the poor. According to filth theory, any number of contagious diseases were caused by bad air that had been made foul by excrement or rot. The sanitary conditions of the urban poor threatened the middle class, who shuttered their windows against the air blowing off the slums. Filth, it was thought, was responsible not just for disease, but also for immorality. “Unclean! Unclean!” the heroine of Dracula laments when she discovers she has been bitten by the vampire, and her despair is for the fate of her soul as much as the fate of her body.