What is the model for infectious disease made of the external agent, a susceptible host, and environmental factors?

• A. Epidemiologic Triad
• B. Chain of Infection
• C. Port of Entry
• D. Transmission Cycle

Answer: (A)

The Epidemiologic Triad shows disease arising from the interaction of three elements: a pathogen (external agent) capable of causing disease, a susceptible host, and environmental factors that influence exposure and transmission. This model highlights how each component contributes and how changing any one can prevent infection. For instance, reducing host susceptibility through vaccination or immunity, removing or weakening the agent through disinfection or treatment, or altering the environment to limit contact or transmission (like improving sanitation, ventilation, or vector control) can all break the chain of infection. The other concepts describe the process of transmission in different ways—the chain of infection outlines steps a pathogen takes to spread, a portal of entry is one specific route, and a transmission cycle describes how disease moves through populations—yet none center on the trio interaction the way the Epidemiologic Triad does.