We can consider this remarkable decline as three distinct periods each corresponding to a dominant transmission pattern. In the first period, from 1912 to 1917, incidence rates exceeded 63 per 100,000 per year that is each year, typhoid fever struck at least 1 in every 1,600 persons in the United States. This period ended abruptly with a drop in incidence rates to between 50 and 3.5 cases per 100,000 by 1950. What happened?

Time when water treatment plants were build in the United States. Large ongoing water borne epidemics ceased being a major route of transmission of S. typhi in the US