The spatial distribution of pertussis, but not measles or smallpox, in pre-industrial Finland matches dialects.

Infections spreading from host to host are a burden of social lifestyle mostly documented at the local scale (within groups). The influence of social structure at a broader scale (e.g., between groups or regions) on infectious disease dynamics is less understood partly due to the difficulty to identify the relevant social groups at this scale. Dialect groups encompass long-held human contacts and could indicate social groups relevant to infections. Using nationwide individual-level mortality records from pre-industrial Finland (1800-1850), we investigated which social grouping best predicted spatial variation in smallpox, pertussis, and measles mortality by comparing models with no regional information, administrative regions, and dialect groups. Dialect groups explained spatial variation of pertussis, administrative regions for smallpox, while measles showed no broader scale spatial variation. These results highlight the complex spatial structuring of infectious diseases and stress the need for studies to identify the relevant social structure.