When is a parasite considered a vector?

A parasite is considered a vector when it actively transmits pathogens from one host to another, facilitating the spread of disease. In this context, the parasite typically carries the infectious agent—such as a virus, bacterium, or protozoan—while also benefiting from its relationship with the host, often without causing immediate harm to the vector itself. Examples include mosquitoes that transmit malaria or dengue fever. Thus, the distinction lies in the parasite's role in disease transmission rather than merely being a host.