The Hägerstrand Model traces the diffusion process by imitating it with numbers.
The model is based on probability rather than fixed numbers.

There are several important concepts in the model:

The probability of contact between a carrier and a non-carrier is determined by the number of people living within a 5x5km square. This is called the neighbourhood effect.
The number of people infected by a disease over time follows an S-shaped curve.
The process of diffusion can be stopped by physical barriers.