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Cities across the United States exhibited different timing in terms of the decline in typhoid mortality during the late 19th and early 20th centuries, and also exhibited very different patterns of seasonality that did not follow a clear geographic pattern. For example, typhoid dynamics in New York are characterized by strong seasonal oscillations and a gradual decline in mortality beginning around 1900, while Philadelphia exhibits multiannual typhoid oscillations with no clear seasonality and higher overall incidence rate prior to the staged introduction of filtration and chlorination beginning in 1904. We are using mathematical models to examine the correlation between data on financial investments into the water supply and sanitation systems and changes in the transmission rate estimated by fitting dynamic models to the typhoid mortality data. This approach will also allow us to quantify the degree of herd immunity exhibited by typhoid dynamics and gain a better understanding of the underlying epidemiology.