Which of the following is NOT a link in the chain of infection?

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

Answer: (D)

The chain of infection describes how a disease passes from one person to another: the infectious agent (pathogen), a reservoir where it can live, a portal of exit for the pathogen, a mode of transmission, a portal of entry into a new host, and a susceptible host who can be infected. A vaccine doesn’t fit into this sequence because it isn’t a pathogen, a place for the pathogen to live, an exit route, a means of transmission, or an entry route. Instead, a vaccine is a preventive measure that strengthens immunity, reducing the chance that exposure leads to infection. Therefore, vaccine is the one that does not belong in the chain.