Supplementary Text 2: Cultural Differences

Supplementary Text 2: Cultural differences

In analyzing data collected and published by the military, we were able to minimize confounding factors, such as demography and socio-economic status.

Soldiers were more homogenous demographically than the general population, and health care, both diagnosis and therapy, were more consistent than in the general population. The most publicized differences between the regiments were lower pay for

African American soldiers until 1864 and the discrimination against non-Caucasians for officer positions(2), neither of which we believe had a large effect on YF outcome.

Differences in food or living conditions could conceivably affect the case fatality rates, but Union war policy prescribed that all soldiers received the same rations (save a brief time in 1863)(2, 30) and equipment for establishing camp(2). Judging by most surviving letters from soldiers, these policies were enforced relatively well(2). The care received at segregated hospitals in the early 1860s(17) could have affected the case fatality rates from diseases, however, the clear clinical features of YF and lack of effective treatment precludes us from stating that this factor explains the entire 2.8 fold difference in case fatality rates. In conclusion, YF case fatality is more skewed towards Caucasians than other infectious disease among military personnel, which suggests a unique explanation unlikely to be based on socio-economic status. An exception to this was the higher case fatality rates from smallpox for Caucasians than non-Caucasians in 1864. This may be an unusual year, as analysis of smallpox case fatalities from 1865 indicated that the case fatality rates between the two populations were not statistically significant

(Supplementary Table 5), and records(17, 25) suggest there were a greater proportion of deaths due to smallpox in non-Caucasians than Caucasians during the Civil War. Of the cultural differences between Caucasians and non-Caucasians that we considered, religion had the potential to affect the case fatality rates, as religious organizations played a major role in organizing donations and medical care during some of the 19th century US epidemics(20). There was and continues to be no specific treatment for YF, so even if one of the populations had increased access to the resources of religious organizations, it would not explain the skewed rates in case fatality. If another potentially confounding factor had affected the rates, such as the effect of traditional medicine sometimes practiced by those of African descent at the time, this probably would have affected trends for other diseases (but as Figure 2 demonstrates, this did not occur). We cannot, however, rule out that non-Caucasians had an effective traditional remedy for YF, but we argue this is very unlikely.