Which model explains disease risk as the result of interactions among an external agent, a susceptible host, and the environment?

• A. Agent-Host-Environment Model
• B. Health Belief Model
• C. Health Promotion Model
• D. Stages of Change Model

Answer: (A)

At the heart of this concept is the idea that disease risk comes from how three components interact: an external agent, a susceptible host, and the environment. This is the epidemiologic triad, which explains why a disease occurs when a pathogen or harmful factor is present (the agent), the person exposed is vulnerable (host), and the surroundings or conditions that facilitate exposure are in place (environment). For example, exposure to a contagious bacterium in crowded, poorly ventilated spaces raises transmission risk, especially if the person has a weakened immune system. This model also shows how reducing risk can target any of the three areas—limiting exposure, boosting host defenses, or improving environmental conditions. The other models mentioned focus on how people think about health, their beliefs, or stages of readiness to change behavior, rather than explaining how disease risk arises from the interaction of agent, host, and environment.