Months after seeing the Prince of Wales through the most sensationalized case of typhoid fever in British history, William Gull delivered a prophetic lecture on the disease at Guy’s Hospital in June of 1872. Calling typhoid by its colloquial name, the “filth fever,” Gull foresaw that in 250 years typhoid would be “uncommon” and “comparatively rare.” Coming off of his successful treatment of the prince, Gull, aged fifty-seven, had an optimistic view that typhoid could be conquered. But the defeat of typhoid, according to Gull, was going to take more than the cautious clinical case management he showed at Sandringham. He went so far as to say that there was “no use tinkering with the disease if one does not try to prevent it, and it no doubt may be prevented.” Prevention, Gull argued, was predicated upon knowledge of the etiology of typhoid. “The theory,” he went on, “is that it is connected with germs which get into the blood; we know nothing about these germs—the air is full of them. There is an idea that they are imbibed by drinking water, and that they increase and multiply within the body.” “Although this has not been demonstrated,” Gull explained, “it is a good working theory.” By the early 1870s it was well accepted that typhoid was waterborne, even if, as Gull noted, the theory still needed further evidentiary proof. Sewer gases, as explored in chapter 1, had gained unprecedented attention as a medium of the disease after the sickness of the Prince of Wales. And as we will see in chapter 3, cow’s milk exploded onto the scene as a conveyor of typhoid in 1871, expanding the epidemiological picture of the disease. But there were plenty of unanswered questions: about the nature of the “poison,” or “germ”; about the alleged spontaneous origin of the disease; and about the uneven distribution of typhoid geographically and demographically. These queries would largely be settled in the last three decades of the nineteenth century through a combination of laboratory study, statistical analysis, and local outbreak investigation, which was spearheaded at the Medical Department.

This chapter explores the development of what Gull called “a good working theory.” To the vanguard of British epidemiologists, Gull’s working theory was at the center of a practical methodology of outbreak investigation.