An Important Point About the Agent-Host-Environment Model (The Epidemiologic Triangle)

An important point about the agent-host-environment model (the epidemiologic triangle) of epidemiologic investigation is that:

A) the agent is not as important a factor in disease causality as the host or the environment.
B) the etiology of infectious disease involves more complex multivariate causality as well.
C) it is necessary that the agent be linked to both the host and the environment and that the agent is present in every case of the disease.
D) in order to prove causality, which is a difficult task, the researcher must prove that the agent, host, and environmental factors under investigation are both necessary and sufficient to cause the disease being studied.