What are the three parts of the epidemiologic triad?

• A. Host, Agent, Environment
• B. Host, Vector, Environment
• C. Agent, Vector, Host
• D. Environment, Host, Time

Answer: (A)

Disease occurs when an infectious agent interacts with a susceptible host in an environment that supports transmission. The agent is the biological cause of the disease (bacteria, virus, parasite, etc.). The host is the person or animal that can become infected and may or may not develop illness, with susceptibility and behaviors influencing risk. The environment encompasses external factors that affect exposure and transmission—habitat, climate, sanitation, population density, vectors, and other conditions that enable spread. An outbreak requires all three elements to be present in the right context. Time influences the course and timing of transmission, but it is not one of the three components of the triad. Therefore, the three parts are agent, host, and environment.