IDENTIFICATION OF PATTERN AND PROCESS IN THE SPREAD OF A CONTAGIOUS-DISEASE

This study investigates spatial and functional aspects of the diffusion of a contagious disease. An epidemic of variola minor (the mild form of smallpox) was sharply circumscribed to a Brazilian county in 1956, after importation from another state. The locations and dates of the onset of 485 recorded cases and the chain of person-to-person transmission of the disease are analyzed here. Using geographers' conceptualizations and techniques, a spatial pattern including neighborhood, hierarchical, random and mixed modes of diffusion have been identified on cadastral maps. The spread through a hierarchical network of social communications is schematized as occurring at three levels. The highest level corresponds to two geographically and socially distinct communities (the city and the rural parts of the county) with few interactions between them. The second level consists of more frequent zonal interactions among distinct city subdivisions, rural districts and the city's elementary day schools. The latter have been shown to act as well-defined diffusion agencies which clearly multiplied the rate of epidemic progression and the affected area. The third level consists of frequent interactions between such discrete social groups as households and school classes. Each household (the ultimate setting of the epidemic) was a well-circumscribed cluster of individual cases. The disease diffused through both propagator-supported diffusion (represented by the results of spread in elementary day schools) and non-propagator supported diffusion during one-fourth of the epidemic duration. The latter prevailed before and after that four-month period. © 1979.