In the chain of infection, link six refers to which concept?

• A. Portal of exit
• B. Susceptible host
• C. Reservoir
• D. Modes of transmission

Answer: (B)

Link six refers to the susceptible host—the person whose defenses determine whether infection occurs after exposure. In the chain of infection, after the agent has left its reservoir, and after it has a route of exit and a mode of transmission, it must encounter a host. Whether infection happens depends on the host’s susceptibility: immune status, vaccination, age, chronic illness or malnutrition, skin or mucous membrane integrity, and other factors that affect defenses. If the host is protected or strong enough to resist the pathogen, infection may not occur despite exposure; if the host is vulnerable, infection is more likely. The other concepts—where the agent resides (reservoir), how it leaves (portal of exit), and how it spreads (modes of transmission)—describe different links in the chain and are not the final barrier determined by the host’s susceptibility.