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.

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.

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.