Geographical Risk of Disease Mortality during the US Civil War

Infectious diseases killed more Union soldiers than combat (2:1 ratio) during the US Civil War, 1861-1865, but this mortality was unevenly distributed across 2,000 individual regimental units. Historical disease mortality rates by US state were obtained from old administrative records from the US Civil War. When disease casualties were assigned to individual states on the basis of where the regiments were recruited, distinct geographical patterns emerged, varying by a factor of 10. More rural areas experienced higher disease mortality rates, especially in southern and border states. A strong correlation (R2 = 0.58; P <0.01) between Civil War disease mortality and the proportion of farming occupations by state was found by using data from the 1860 Census. Low population density also correlated with higher disease mortality rates, but the area of farmland or wheat production in 1860 did not. Elevated disease mortality among Black and Indian (Native American) soldiers was comparable to that of White Union soldiers recruited from the Confederate states and was much higher than that of soldiers from more urban states in the Northern and Eastern regions. Disease mortality during the US Civil War varied greatly between groups, with epidemiological isolation prior to the war associated with higher mortality and urban living or prior military service associated with lower mortality. This difference is likely due to varied immunity to infectious pathogens.