The syllabus says
- Explain how the geographic concepts of diffusion by relocation and by expansion apply to the spread of diseases. Examine the application of the concept of barriers in attempts to limit the spread of diseases. Describe the factors that have enabled reduction in incidence of a disease.
Key terms
- Diffusion - how something (such as a disease) spreads out. It can be spatial or through a population
- Diffusion by relocation - the spreading disease moves into new areas, leaving the origin behind
- Diffusion by expansion - the spreading disease moves into new areas while remaining in the original source
- Barrier (to disease) - a factor that prevents the disease spreading either within a population or to a new location
- Prevalence - the number of cases per 10,000 population.
- Incidence - the total number of cases per year.
- Epidemic - an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area
- Pandemic -an epidemic that occurs worldwide/ over a very large area
Pandemic, epidemic, or outbreak?
These terms are commonly confused. The level of disease normally present in a population is known as the 'endemic' level of disease. Therefore epidemic, outbreak and pandemic relate to elevated levels of a disease. Note that the severity of the disease is not part of the definition for any of these key terms - so epidemics, outbreaks and pandemics can be for relatively mild symptoms or can cause widespread death.
An epidemic is defined as "an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area".
This is very similar to outbreak which the WHO defines as: "A disease outbreak is the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season. An outbreak may occur in a restricted geographical area, or may extend over several countries. It may last for a few days or weeks, or for several years." Note, however, that an outbreak is restricted spatially, while the epidemic may extend over a wider spatial area.
The WHO defines pandemic as: 'an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people'. This has been accused of being too vague by bodies such as the Council of Europe but the WHO defends its definition because its use by specialists is dependent on specific conditions including that a
"true pandemic occurs when almost simultaneous transmission takes place worldwide".
An epidemic is defined as "an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area".
This is very similar to outbreak which the WHO defines as: "A disease outbreak is the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season. An outbreak may occur in a restricted geographical area, or may extend over several countries. It may last for a few days or weeks, or for several years." Note, however, that an outbreak is restricted spatially, while the epidemic may extend over a wider spatial area.
The WHO defines pandemic as: 'an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people'. This has been accused of being too vague by bodies such as the Council of Europe but the WHO defends its definition because its use by specialists is dependent on specific conditions including that a
"true pandemic occurs when almost simultaneous transmission takes place worldwide".
Types of diffusion
The diffusion of a disease refers to how it spreads. This can be through space (spatial diffusion) or heirarchical (through a population e.g. attacking vulnerable people first). Diffusion occurs in several patterns, which are described here. However, check the types that are relevant to your studies (see syllabus content above).
The main types of diffusion that relate to disease are expansion diffusion and relocation diffusion. The animation below demonstrates how these operate for disease. Note how most diseases spread through a combination of diffusion types, rather than one single type.
The main types of diffusion that relate to disease are expansion diffusion and relocation diffusion. The animation below demonstrates how these operate for disease. Note how most diseases spread through a combination of diffusion types, rather than one single type.
Expansion diffusion
Expansion diffusion involves the disease spreading from a central source. "The spreading phenomena [of a disease] has a source and diffuses outwards into new area" (source) gradually. There is no 'jump' in the disease as it moves to a new area. HIV is a disease spread by expansion because the disease remains in its original location as well as spreading out from the source. This is because the source of the disease (Patient Zero) is not cured. Relocation diffusion Relocation diffusion "describes the spread that occurs when the spreading phenomena moves into new areas, but leaves behind its origin or source" (ibid). In terms of how disease spreads in this way amongst humans it involves the movement of infectious people to jump into new areas (source) usually by a means of modern transport. It is therefore also known as 'transfer diffusion'. |
Source: http://www.makingthemodernworld.org.uk/learning_modules/geography/05.TU.01/?section=7
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The British Geographer website has a great section on this issue. The diagrams below, taken from that site, show how these types of diffusion occur:
Other types of diffusion
The information below is taken directly from Wikipedia:
Note that contagious diffusion only applies to direct contact of infected individuals, so diseases such as herpes are examples of contagious diffusion while the common cold can be spread through the air so it is more likely to be expansion diffusion (though direct contact can also spread it).
The information below is taken directly from Wikipedia:
- "Contagious diffusion is the spread of an infectious disease through the direct contact of individuals with those infected.
- "Hierarchal diffusion occurs when a phenomenon spreads through an ordered sequence of classes or places.
- "Network diffusion occurs when a disease spreads via transportation and social networks, “reflecting the geographical and social structuring of human interactions”.
- "Mixed diffusion is a combination of contagious diffusion and hierarchal diffusion. AIDS is a prominent example in modern day society of a mixed diffusion disease, often spreading along the hierarchal, network, and contagious diffusion patterns."
Note that contagious diffusion only applies to direct contact of infected individuals, so diseases such as herpes are examples of contagious diffusion while the common cold can be spread through the air so it is more likely to be expansion diffusion (though direct contact can also spread it).
Barriers to Disease
Geographical barriers to disease can be split into human and physical. While human barriers can prevent the diffusion of disease both spatially and within a population, physical barriers usually only restrict the disease spatially.
Human barriers
Physical barriers
These largely limit human movement, but can also limit the spread of the vector (carrier) of the disease, such as mosquitos. The main barrier is distance. The theory of distance decay and friction of distance are strongly linked here.
Human barriers
- Quarantine
- Lack of legal entry (e.g. no HIV patients are allowed to enter Malaysia, although there is no requirement to be tested before entry)
- School closure to avoid spread
- Personal protection e.g. malaria nets and practicing safe sex using barrier contraception
- Public information campaigns
Physical barriers
These largely limit human movement, but can also limit the spread of the vector (carrier) of the disease, such as mosquitos. The main barrier is distance. The theory of distance decay and friction of distance are strongly linked here.
- Sea and ocean prevents spread of disease between people
- Mountains and other inhospitable environments prevent the spread of disease by vectors e.g. mosquitos can't cross deserts
factors reducing incidence of disease
These are dealt with in the specific case studies on this site.