Which term describes a person or animal that harbors an infectious agent without discernible disease who can serve as a source of infection?

• A. Carrier
• B. Reservoir
• C. Vector
• D. Host

Answer: (A)

The concept being tested is asymptomatic carriage and the ability to transmit infection. A carrier is someone (or an animal) who harbors an infectious agent without showing discernible disease but can still shed the pathogen and spread it to others. This distinction matters because transmission can occur even when there are no symptoms, making carriers important sources of infection in a population.

Why this fits best: it specifically describes an individual who is harboring the agent and is capable of transmission despite the absence of illness. The term emphasizes the person or animal as the source of infection through ongoing shedding or contact with others, rather than focusing on where the agent resides, the intermediary that moves it between hosts, or the broader host category.

Why the other terms are not as exact: a reservoir is the natural habitat or population where the agent persists over time, not necessarily a single asymptomatic carrier who actively transmits to others. A vector is a living intermediary that transmits the pathogen between hosts, such as a mosquito, but the term describes the mechanism of transmission rather than an infected individual who harbors the agent. A host is a broad term for any organism that can be infected, which may include symptomatic cases; it does not inherently specify asymptomatic carriage with transmission potential.

So, the term that captures an asymptomatic individual who can serve as a source of infection is carrier.