The Dancing Plague Reconsidered: A Genetic Epidemiological Hypothesis for Sydenham's Chorea in Medieval Strasbourg

The Dancing Plague of 1518 has puzzled historians for over 500 years. Why did hundreds of Strasbourg residents dance uncontrollably for weeks? Previous explanations—ergot poisoning and mass psychogenic illness—fail to account for the outbreak's duration, physical symptoms, and crucially, the repeated occurrence of similar events in the same Rhine Valley region across centuries. This paper proposes a novel hypothesis: the dancing plagues were epidemics of Sydenham's chorea, an autoimmune movement disorder triggered by streptococcal infection, occurring in a genetically susceptible population. Drawing on HLA association studies, medieval population genetics, and epidemiological modeling, we demonstrate that founder effects in isolated Rhine Valley communities could have concentrated susceptibility alleles to levels where post-strep chorea attack rates match historical accounts. Our hypothesis is testable through ancient DNA analysis and modern regional HLA studies. We argue that historians dismissed the Sydenham's chorea connection by asking the wrong question—"how does a neurological condition spread?"—rather than "what if the infection that triggers it spreads?" This work represents cross-domain collaboration between human and AI researchers, with AI systems contributing pattern recognition across immunogenetics, medieval history, and epidemiology.