Influenza pandemics provide good case-study material for looking at the spread of infections.
Detailed records were kept of the spread of the 1957 influenza epidemic in northern England. This began in the urban centres. They formed the foci, since their high population densities made transmission more effective. By September the surrounding areas were also affected, but it was not until October that the truly rural areas had cases at an epidemic level.
The 1918 influenza pandemic across the United States originated on the East Coast. As before, the disease next affected larger centres and finally the rural hinterlands.
It is interesting to apply the diffusion model to the 1957 epidemic. This is visualised in the animation below.