Disease diffusion occurs when a disease is transmitted to a new location.[1] It implies that a disease spreads, or pours out, from a central source.[2] The idea of showing the spread of disease using a diffusion pattern is relatively modern, compared to earlier methods of mapping disease, which are still used today.[3] According to Rytokonen, the goals of disease mapping are: 1) to describe the spatial variation in disease incidence to formulate an etiological hypothesis; 2) to identify areas of high risk in order to increase prevention; and 3) to provide a map of disease risk for a region for better risk preparedness.[4]
Torsten Hägerstrand’s early work on “waves of innovation” is the basis that many medical cartographers and geographers use for mapping spatial diffusion (1968).[5] The diffusion of disease can be described in four patterns: expansion diffusion, contagious diffusion, hierarchal diffusion and relocation diffusion.[6] Cromley and McLafferty also mention network diffusion and mixed diffusion.[1]
The diffusion of infectious disease tends to occur in a ‘wave’ fashion, spreading from a central source. Pyle mentions barriers that pose a resistance towards a wave of diffusion, which include but are not limited to: physiographic features (i.e. mountains, water bodies), political boundaries, linguistic barriers, and with diseases, a barrier could be differing control programs.[7] The diffusion of disease can be identified as a normal distribution over time and translated into an S-shaped curve to show the phases of disease diffusion. The phases are: Infusion (25th percentile), Inflection (50th percentile), Saturation (75th percentile), and Waning to the upper limits.[8]