Jefferson was primarily settled by French-Canadians. By 1867 the town consisted of 25 or so French-Canadian families.http://www.waymarking.com/waymarks/WMAQ56_St_Peters_Church_Jefferson_SD During the winter of 1880–1, the town was ravaged by a smallpox epidemic, reported to have arrived in the area via Russian Mennonite emigrants en route to Yankton, in the Dakota Territory. The town was quarantined for several weeks, without railroad service or mail delivery. At least 150 cases of "black small pox" were reported by February 1881, resulting in more than 82 deaths."Stricken with Small Pox" New York Times, Feb. 3, 1881 https://timesmachine.nytimes.com/timesmachine/1881/02/03/103396017.pdf
