Disease Diffusion and Epidemiological Geography

Medical geography treats disease not as a purely biological event but as a spatial process, one that moves across the earth's surface following patterns that can be mapped, modeled, and predicted. The foundational distinction, formalized by the geographer Peter Haggett in Spatial Diffusion (1968, with Torsten Hägerstrand's earlier 1953 work on innovation diffusion as its direct ancestor), separates expansion diffusion, in which a disease spreads outward from a source area while also persisting there, from relocation diffusion, in which the disease is carried to a new area and the original source may fade. Expansion diffusion further divides into contagious diffusion, which depends on direct contact and therefore weakens with distance (as in most respiratory and contact-transmitted infections), and hierarchical diffusion, which jumps between large, well-connected population centers before filtering down to smaller towns — the pattern by which HIV/AIDS moved between major world cities in the 1980s well ahead of its arrival in the rural areas between them. John Snow's 1854 investigation of the Broad Street cholera outbreak in London's Soho district is conventionally treated as the founding case study of the field: by plotting cholera deaths on a street map and tracing them to a single contaminated water pump, Snow demonstrated a waterborne mechanism of transmission decades before Robert Koch isolated Vibrio cholerae in 1883, and his dot map remains a standard teaching example of spatial epidemiological reasoning.