Throughout life, an individual's state of health continually changes between wellness and illness. Illness prevention is the term used for a range of approaches to reduce the risk of ill health and promote good health.

The health-illness continuum model can be visually represented with a two-way arrow showing health and illness on each side.

Movement towards the right symbolizes achieving a higher level of health, which may be facilitated by health awareness, education, and growth.

In contrast, movement towards the left symbolizes a decline in an individual's health, which can progress from early signs of potential illness, to symptoms, then disability, and, ultimately, premature death.    

In the agent-host-environment model, disease results from the interaction between an agent and a susceptible host in an environment.

In this triad, the agent is the cause of disease. The host is an organism, usually a human or an animal, that harbors the disease, and the environment is the surrounding that causes or allows disease transmission.

In this model, health is maintained when there is a balance between the three components.