Interdisciplinary approaches in epidemic studies--II: Four geographic models of the flow of contagious disease

Straightforward adaptation of geographical and regional-science models to conceptual modeling of the epidemic spread of a contagious disease is achieved by: (a) using analogies and isomorphisms, and (b) using as a real example the observations made during a well studied epidemic of variola minor (the mild form of smallpox), a typical contagious disease. The adaptation of the Wilson model of planning for urban development includes (a) a static view (network) of the structure and activities of the population and of organizations (diffusion agencies such as day schools), and (b) the dynamic view (mechanism of epidemic spread) which includes the changes with time of elements of the network brought by flow of disease. Adaptation of the Brown model of spatial diffusion yields flows of disease occurring between micro-scale units (households) of social interactions but aggregations of these units do not participate explicitly in the flows. The occurence of successive generations of affected households is specified as well as the occurence of definite stages of the epidemic progression. Adaptation of the Alves-Morrill model of spatial diffusion yields (a) a network of social groups and interdependencies; (b) a simplified network of the mechanism of spread that shows flows of disease between individuals grouped into generations of infected individuals and generations of infective individuals; and (c) a more realistic view which shows the flows of disease between real epidemiological units such as households and school classes. Adaptation of the Morrill-Manninen model of spatial diffusion concentrates on the mechanism and parameters having the epidemic spread as output. The interdependencies between the parameters and between each parameter and the epidemic spread are represented, including feedback processes. Brown's model seems to be the best for describing the epidemic spread of contagious disease while the Morrill-Manninen model is the most promising for investigating the detailed mechanism of the spread. Since these two models complement each other, their combined use is indicated.