Describe the chain of infection and its six components.

• A. Agent, reservoir, portal of exit, transmission, portal of entry, susceptible host.
• B. Host, agent, transmission, reservoir, portal of exit, portal of entry.
• C. Reservoir, portal of exit, portal of entry, transmission, agent, host.
• D. Transmission, agent, reservoir, host, portal of exit, portal of entry.

Answer: (A)

The chain of infection describes how a pathogen moves from one person to another and how we can interrupt that flow at different points. It starts with the agent, the microorganism capable of causing disease. The agent must have a reservoir, a place where it can survive and multiply. From there, it exits through a portal of exit, such as the respiratory tract or a blood source. It then transfers to a new host via a mode of transmission—direct contact, droplets, airborne spread, a vehicle, or a vector. Once it reaches a new host, it must gain entry through a portal of entry, like mucous membranes, broken skin, or the gastrointestinal tract. Finally, the host must be susceptible, meaning lacking immunity or being vulnerable to infection.

That sequence—agent, reservoir, portal of exit, transmission, portal of entry, susceptible host—is the standard order used to describe how infections spread. The other options mix up the order or place the host or transmission steps in the wrong place, which doesn’t reflect how the chain progresses from pathogen to susceptible person. Understanding this helps identify where interventions (like isolation, hand hygiene, masking, vaccination) can effectively break the chain.