Which term describes an organism that spreads disease between hosts?

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

Answer: (D)

Transferring disease from one host to another relies on a mediator that carries and passes the pathogen between individuals. The term that fits this role is vector—usually a living organism like a mosquito, tick, or flea that acquires a pathogen from an infected host and transmits it to a new host, often through a bite or contaminated contact. This differs from a reservoir, which is where the pathogen persists (in an environment or another organism) without actively moving it to new hosts at that moment, and from a host, which is the organism that becomes infected. A carrier harbors and can shed the pathogen, but the explicit link in spreading between hosts in many cycles is captured by the vector. Thus, the organism spreading disease between hosts is a vector.