What term is used for a living carrier that transmits disease?

The term that describes a living carrier that transmits disease is "vector." Vectors are typically organisms, such as insects, that can carry pathogens from one host to another, facilitating the spread of disease. For instance, mosquitoes act as vectors for malaria and dengue fever by transmitting parasites or viruses to humans during their feeding process.

In contrast, a pathogen refers to the microorganism itself that causes disease, such as bacteria, viruses, fungi, or parasites, but does not describe the carrier. An antigen is a substance that triggers an immune response, which is also unrelated to the concept of a living carrier. Lastly, a parasite signifies an organism that lives on or in a host and benefits by deriving nutrients at the host's expense, but it does not specifically denote a carrier that transmits diseases from one host to another. Therefore, vector is the most accurate term for a living carrier that transmits disease.