Which sequence correctly lists the components of the chain of infection?

• A. Infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host
• B. Reservoir, portal of exit, mode of transmission, infectious agent, portal of entry, susceptible host
• C. Portal of exit, mode of transmission, portal of entry, infectious agent, reservoir, susceptible host
• D. Susceptible host, infectious agent, reservoir, portal of exit, mode of transmission, portal of entry

Answer: (A)

The sequence starts with the infectious agent—the microorganism capable of causing disease. From there, it needs a reservoir, which is the place where it can survive or multiply (humans, animals, or the environment). It then exits the reservoir through a portal of exit (such as the respiratory tract, blood, or gastrointestinal tract). After exiting, the agent is transmitted to another person via a mode of transmission (direct contact, droplets, airborne spread, vehicles, or vectors). Once outside the source, it must find a portal of entry to enter a new host (mucous membranes, broken skin, etc.). Finally, a susceptible host is required for infection to occur, meaning someone without immunity or with increased vulnerability. This logical progression is exactly captured by listing infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host. Other sequences disrupt this flow by placing components out of order (for example, placing the agent after transmission or entry before exit), which is why they don’t fit.