For 6a, the 3 elements of the epi triangle are not presented specific to each country.
Epidemiologic triangle, web of causality, and the ecological model Epidemiologists understand that disease
results from complex relationships among causal agents, susceptible persons, and environmental factors.
These three elements—agent, host, and environment—are traditionally referred to as the epidemiologic
triangle (Fig. 13.2A). This model originally was developed as a way of identifying causative factors,
transmission, and risk related to infectious diseases. Changes in one of the elements of the triangle can
influence the occurrence of disease by increasing or decreasing a person’s risk for disease. As illustrated in
Fig. 13.2B, specific characteristics of agent and host, as well as the interactions between agent and host, are
influenced by the environmental context in which they exist, and may in turn influence the environment. Box
13.1 provides examples of these three components. FIG. 13.2A and B, Two models of the agent-hostenvironment interaction (the epidemiologic triangle). BOX 13.1 Examples of Agent, Host, and Environmental
Factors in the Epidemiologic Triangle Agent Infectious agents (e.g., bacteria, viruses, fungi, parasites)
Chemical agents (e.g., heavy metals, toxic chemicals, pesticides) Physical agents (e.g., radiation, heat, cold,
machinery) Host Genetic susceptibility Immutable characteristics (e.g., age, sex) Acquired characteristics (e.g.,
immunological status) Lifestyle factors (e.g., diet and exercise) Environment Climate (e.g., temperature,
rainfall) Plant and animal life (e.g., agents or reservoirs or habitats for agents) Human population distribution
(e.g., crowding, social support) Socioeconomic factors (e.g., education, resources, access to care) Working
conditions (e.g., levels of stress, noise, satisfaction)
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