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 Agent, Host and Environment Model of Causation   To Prevention theme page

1. Core Knowledge

This triangular model was developed for infectious disease. Disease spread requires a susceptible host and an infective agent, in an environment that brings them together:

A triangle links agent, host and envrionmental factors, each of which contributes to the likelihood of disease.

Note that the model is also helpful in thinking about non-communicable diseases:


Applying the triad model to prevention, to prevent the disease it should be sufficient to modify one (but preferably more than one) of the factors on the corners of the triangle: to immunize the host, or to develop a protective environment, or to remove the agent. These fall under health protection or hazard control measures.

If you cannot remove or modify the corners of the triangle you can also prevent disease by breaking the lines: remove the connection between the agent and the environment (e.g. install window screens to keep malarial mosquitoes out of the house); protect the host from coming into contact with the agent (use bug spray or mosquito nets); move the host to a different environment (leave town!). This idea is taken up in the Source - Path - Receiver model in occupational health.