Hägerstrand's diffusion curve is a model created by Swedish geographer Torsten Hägerstrand to study the spread of innovations. Key points:

This model factors in elements such as information regarding innovations, financial security, the psychological make-up of the adopter, and physical proximity to other adopters.

At first, few people adopt an innovation. As information becomes more widespread and the cost is reduced, more people adopt the innovation. However, some people resist change, causing inequalities in adoption.

Example: Imagine a new smartphone app that helps people learn foreign languages. Initially, only tech enthusiasts and language learners may adopt it. As its benefits become more widely known and the app becomes free or cheaper, more people start using it. Yet, those who are not comfortable with tech or do not see the need to learn a new language might never adopt this innovation.

Disease diffusion refers to how diseases spread into new locations. Main points:

Disease spread usually follows a "wave" pattern, spreading out from a central source. There are four main patterns of disease diffusion: Expansion, Relocation, Contagious, Hierarchical, along with Network and Mixed diffusion. Physical and economic barriers, as well as political boundaries, can limit disease spread. The spread of disease can be visualized as an S-shaped curve with four phases: Infusion, Inflection, Saturation, and Waning.

Example: The COVID-19 pandemic spread globally through a mix of these diffusion types. It spread through direct contact (Contagious diffusion), moved to new areas as people traveled (Relocation diffusion), spread to new countries following transportation routes and social networks (Network diffusion), and spread from major cities to smaller ones (Hierarchical diffusion).

Zika virus arrived in Brazil in 2014 and has since spread to 21 other countries in the Americas.

The virus is primarily mosquito-borne but can also be transmitted through sexual contact. It causes mild symptoms in most infected people, but it can cause birth defects in children and neurological problems in adults.

The spread of Zika is an example of relocation diffusion as people traveled to and from affected areas, unknowingly transporting the virus.