George Soper was not your typical detective. He was a civil engineer by training, but had become something of an expert in sanitation. So when, in 1906, a landlord in Long Island was struggling to trace the source of a typhoid outbreak, Soper was called in. The landlord had rented his Long Island house to a banker’s family and servants that summer. By late August, six of the house’s 11 inhabitants had fallen ill with typhoid fever.
Soper had been previously hired by New York state to investigate disease outbreaks—“I was called an epidemic fighter,” he later wrote—and believed that typhoid could be spread by one person serving as a carrier. In Long Island, he focused his attention on the cook, Mary Mallon, who had arrived three weeks before the first person became ill.
What Soper discovered would demonstrate how an unwitting carrier could be the root of disease outbreaks, and, later, spark a debate about personal autonomy when it’s pitted against public health.
Combing through the roster of wealthy New Yorkers who had employed Mallon in their summer homes...