Which of the following is NOT part of the chain of infection?

• A. Infectious Agent
• B. Portal of Exit
• C. Reservoir
• D. Vaccine

Answer: (D)

The chain of infection describes how disease spreads through six interconnected links: the infectious agent, a reservoir, a portal of exit, a mode of transmission, a portal of entry, and a susceptible host. A vaccine is not one of these links; it’s a preventive measure. By boosting immunity, a vaccine reduces the chance that a susceptible person becomes infected or that transmission occurs, effectively breaking the chain before infection takes hold. The infectious agent, reservoir, and portal of exit are all part of the chain because they describe what the pathogen is, where it lives, and how it leaves to reach a new host.