Diffusion models attempt to show different ways in which infection can spread out from a central point. There are two main kinds of this 'expansion diffusion':
- 'Contagious diffusion' is infection spread by direct contact. This means that the risk of being infected is lessened with distance. A measles epidemic is a clear example.
- 'Hierarchic diffusion' is infection spread through a class or group, for instance where it begins in a large settlement and gradually spreads out to progressively smaller ones. The spread of HIV/AIDS from larger to smaller centres in the United States would be an example of this.
These two types of expansion diffusion can also work together.
There is also 'relocation diffusion', which occurs when an infection spreads into a new area but leaves its source behind. Some historical influenza epidemics have taken this form.