Contagion Models 1 Diffusion

Diffusion models describe state changes through specified contacts and transmission rules. A simple SI model contains susceptible and infected states without recovery.

A diffusion model specifies states, contacts, and a rule by which something spreads. A simple susceptible–infected model allows a susceptible unit to become infected through contact and does not include recovery. It can illustrate how growth initially accelerates and later slows as fewer susceptible units remain, under the chosen mixing and transmission assumptions.

Recovery, immunity, external exposure, and social reinforcement require additional states or mechanisms. A rumor, product, and pathogen need not obey the same contact rule. Nor does a reproduction number above one guarantee that every stochastic introduction grows. Begin with the mechanism and the question being asked, then use the appropriate model rather than treating all spread as one universal equation.

When to use it

When understanding how information, infection, or another state might propagate through a population under explicit assumptions.

How it can help

Define states and contacts, identify missing mechanisms, and compare a model’s predictions with the actual spreading process.