Which of the following is NOT a component of the epidemiologic triad?

• A. Transmission
• B. Host
• C. Agent
• D. Environment

Answer: (A)

The key idea here is what the epidemiologic triad consists of. The triad focuses on three elements that must interact for disease to occur: the agent (the cause), the host (the susceptible organism), and the environment (external factors that affect exposure and spread). Transmission, while essential to how a disease spreads, is the process that moves the agent from one host to another or from the environment to a host. It’s a mechanism, not a pillar of the triad.

For example, malaria involves the agent Plasmodium, the host humans, and an environment with mosquitoes and standing water that supports transmission. The spread of malaria by mosquito bites is transmission, not a separate component of the triad.