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.
Filth theory was eventually replaced by germ theory, a superior understanding of the nature of contagion, but filth theory was not entirely wrong or useless. Raw sewage running in the streets can certainly spread diseases, although smallpox is not one of them, and the sanitation reforms inspired by filth theory dramatically reduced the incidence of cholera, typhus, and plague. Clean drinking water was among the most significant of those reforms. The reversal of the Chicago River, for instance, so that the sewage dumped in the river was not delivered directly to Lake Michigan, the city’s drinking-water supply, had some obvious benefits for the citizens of Chicago.