| Diffusion - structural characteristics - two types:|
"In hierarchical diffusion, ideas leapfrog from one important person to another, or from one urban center to another, temporarily bypassing other persons or rural territory. . . By contrast, contagious diffusion involves the wave-like spread of ideas, without regard to hierarchies, in the manner of contagious disease." (Jordan, Domosh, & Rowntree 1994 The Human Mosaic p.16)
"Like the waves produced by dropping a rock into a pond of water, the phenomenon spreads out through a uniform medium. Contagious diffusion is based on proximity and contact. It is subject to distance decay as intensity attenuates the further you get from the point of origin." (Alan Forsberg)
"The spread of cultural innovation through person to person contact, moving wave-like through an area and population without regard to social status."(Jordan, Domosh, & Rowntree 1994 The Human Mosaic p. G-4 )
"The distance-controlled spreading of idea, innovation, or some other item through a local population by contact from person to person." (DeBlij & Muller Geography: Realms Regions and Concepts 1994 John Wiley & Sons)
At the local scale, a disease like cholera spreads contagiously. For example:
"When mapped out, hierarchical diffusion often produces a series of non-contiguous spots which reflect the spatial expression of a structure - social or physical. (Alan Forsberg, Lecture)
In bioamplification / biomagnification, many toxins accumulate to the highest concentration in the carnivores at the top of the foodchain.
HIV/AIDS is not very contagious. It only spreads through an exchange of bodily fluids. It does not spread evenly, but most often kills the most economically active portion of the population - people in their prime.
Today, people of color and the poor are more likely to suffer from environmental injustice, and more likely to get HIV/AIDS than most Americans.
"Innovations spread from one important person to another or from one urban center to another, temporarily bypassing persons of lesser importance and rural areas." (Jordan, Domosh, & Rowntree 1994 The Human Mosaic p. G-8)
"An idea or innovation spreads by "trickling down" from larger to smaller adoption units. An urban hierarchy is usually involved, encouraging the leapfrogging of innovations over wide areas, with geographic distance a less important influence."(DeBlij & Muller Geography: Realms Regions and Concepts 1994 John Wiley & Sons)
"The process by which contacts between people and the resulting diffusion of things or ideas occurs first among those at the same level of a hierarchy and then among elements at a lower level of the hierarchy" (Getis, Getis & Fellmann 1996 Introduction to Geography)
The difference between expansion diffusion and relocation diffusion has to do with what happens at the point of origin. In expansion diffusion, what is being diffused remains at the point of origin, often becoming more intense. With relocation diffusion, what is being diffused evacuates the point of origin and moves on.