Disease Diffusion

Define Disease diffusion

refers to the spread of a disease into new locations

Define Incidents

the number of infections (but you haven’t died from it)

What is the frictional effect of distance (distance decay)?

This suggests that areas that are closer to the source of disease are more likely to be affected and/or will be affected at a later date

What is the name of the model that is commonly used to predict how disease will spread?

The Hager strand model of diffusion

What are the four types of disease diffusion?

• Expansion diffusion

• Relocation diffusion

• Hierarchical diffusion

• Contagious diffusion

Define Expansion diffusion

when a disease spreads outwards and intensifies in the areas it started. It starts of a new area of prevalence where the disease spreads from.

Name an example of expansion diffusion

Covid 19 - continued to intensify

Define Relocation diffusion

this is where a disease moves to new areas but doesn’t remain in the original area of outbreak

Name an example of relocation diffusion

Cholera from Nepal workers into Haiti

What is the Hearth?

The original area of outbreak

Define Hierarchical diffusion

a disease may be transmitted usually down the urban hierarchy

Name an example of hierarchical diffusion

HIV/AIDs moved to larger cities from rural areas - mainly gay men who were effected in the 1980s

Define Contagious diffusion

where a disease spreads the fasted and is concentrated to those areas that is closest

Name an example of contagious diffusion

The Spanish flu (1918), The Ebola virus (2010s)

What are the first element of the Hager Strand model?

It is probabilistic rather than deterministic

What is the second element of the Hager Strand model?

The number of people infected by an epidemic approximates a S-shaped curve - logistic curve

What are the third element of the Hager Strand model?

The progress of diffusion of a disease may be interrupted by physical barriers

What is the first stage of the Hager Strand model?

Primary stage

What is the second stage of the Hager Strand model?

Expansion stage

What is the third stage of the Hager Strand model?

Condensation stage

What is the forth stage of the Hager Strand model?

Saturated stage

What is the primary step in the graph?

This is when there is a strong contrast in disease incidence between the area of outbreak and more remote areas

What is the expansion step in the graph?

diffusion is centrifugal - new cases of disease outbreak occur at distance from the source and thus reduced the spatial contrast of the primary step

What is the condensation step in the graph?

the number of cases become more stabilised and equal in all locations, irrespective of distance from the source

What is the saturated step in the graph?

Diffusion decelerates as the incidence of the disease reaches its peaks

What is the impact of barriers to diffusion on the model?

It makes it less accurate

What are physical barriers?

Distance - Oceans, climate, desert, mountains

How does distance impact the probability of disease diffusion?

The probability of contagious disease entering an area is inversely proportional to the distance from the source

What is the neighbourhood affect?

people living in close proximity have a greater probability of catching a disease

What are socio-economic/political barriers?

• Screening protocol at borders - USA and Ebola

• UK GPs were educated on Ebola

• Adequate supplies of equipment and PPE

• Quarantine - Covid

• Masks, social distancing, no large distancing

• Vaccination and education

• Travel restrictions

What are the key differences in the response of Ebola and Covid 19?

• There was a slower response to Covid - the UK and US didn’t go into lockdown quickly enough

• Ebola had been around before so it was easier to identity it comparing to Covid

• Ebola had severe symptoms so people could easily be identified and isolates comparing to covid