Disease Diffusion

Defining Diffusion

• Diffusion is the process of diseases spreading outward from their origin and across space.
• This takes several forms:
• ​​​​​​​expansion diffusion
• relocation diffusion
• contagious diffusion
• hierarchical diffusion
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Expansion Diffusion

• Expansion diffusion occurs where a disease starts in one location and spreads out, similar to the "snowball" effect.
• Many cultural practices are spread by a type of cultural diffusion called expansion diffusion.
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Relocation Diffusion

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• Relocation diffusion occurs when a disease completely leaves one area and moves to another.
• An example of this is the cholera epidemic in Haiti in 2010.
• Diseases like this can spread by people flying from one location to another.

Contagious Diffusion

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• Contagious diffusion describes the spread of a disease through direct contact with a carrier.
• An example of this is the Ebola outbreak in Africa in 2014-15.

Hierarchical Diffusion

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• Hierarchical diffusion is diffusion through a sequence of carriers.
• This type of diffusion is usually spread along roads and railways.
• In 2009, the H1N1 virus spread to locations with international airports.

Barriers to Diffusion

• The most important barrier to diffusion is distance. The probability of catching a contagious disease decreases with distance from the source.
• Physical barriers such as mountains, oceans and deserts also stop diffusion.
• Climate can act as a barrier to many tropical diseases.
• Political barriers such as immigration control stop a disease from spreading to the public within a country.
• Mass vaccinations can act as a barrier to diffusion also.

The Hägerstrand Model

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• 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.