In the epidemiologic triad, which component is described as the host or person who may become infected?

• A. Susceptible Host
• B. External Agent
• C. Environmental Factor
• D. Reservoir

Answer: (A)

In the epidemiologic triad, the host is the person or organism that could become infected. When we describe this as a susceptible host, we’re emphasizing that infection depends on the host’s defenses being insufficient to prevent infection after exposure to the agent. Susceptibility is shaped by factors like immunity status, age, nutrition, chronic diseases, vaccination history, and prior exposure. If exposure occurs and the host is susceptible, the agent can invade and cause infection. The other components describe the agent itself (the microorganism or factor capable of causing disease), the environment (conditions that affect transmission and contact), and the reservoir (the source where the agent lives and multiplies). Understanding this helps explain why a disease may spread in some populations but not others: the host’s susceptibility is a key determinant.