Abstract: Regional differences in disease prevalence are associated with a wide array of cross-cultural differences. However, the complex relationships among culture, disease, and other ecological variables remain underinvestigated. Future research into the origins of cultural differences will benefit from the availability of a numerical index identifying the extent to which infectious diseases have been historically prevalent within regions defined by geopolitical borders. This article introduces such an index. This index is based on disease prevalence data obtained from old epidemiological atlases and is calculated for 230 geopolitical regions (mostly nations) around the world.