The chain of infection describes an orderly progression of events in which a disease-causing agent leaves its reservoir through a portal of exit, spreads through a mode of transmission, then enters through an appropriate portal of entry to infect a susceptible host.

• A. An orderly progression of events in which a disease-causing agent leaves its reservoir through a portal of exit, spreads through a mode of transmission, then enters through an appropriate portal of entry to infect a susceptible host.
• B. A process by which immune cells destroy pathogens.
• C. A sequence in which pathogens mutate to become more virulent.
• D. An event where pathogens multiply only inside a host without leaving the reservoir.

Answer: (A)

The chain of infection is about the step-by-step path a pathogen takes to spread from one person to another: it must exit its reservoir through a portal of exit, move through a mode of transmission, and enter a new host through a portal of entry to cause infection. The description in this item matches that sequence exactly by naming each stage—exit from the reservoir, a transmission route, and entry into a susceptible host—so it best captures how infections are transmitted.

The other ideas aren’t about transmission between hosts. Immune cells destroying pathogens describes the body’s defense mechanisms, not how an infectious agent moves between individuals. Pathogens mutating to become more virulent concerns evolutionary changes within or between hosts, not the procedural chain of transmission. Pathogens multiplying only inside a host without leaving the reservoir describes infection without spread to others, which omits the transmission link.

So this option correctly reflects the full, end-to-end process that enables infections to spread and demonstrates why understanding each link is important for interrupting transmission.