A vector is an organism who passes on a disease without getting sick. A vector actively transmits an infectious agent between infected and susceptible vertebrates. Essentially, vectors can transmit infectious agents in two ways. They can serve as a vehicle whereby the infectious agent is conveyed from one host to another without undergoing a stage of development or multiplication. This is known as mechanical transmission. Alternatively, the infectious agent can undergo some stage of development or multiplication in the vector - this is known as biological transmission.
The term carrier is used to describe an individual that is infected by a disease agent and is capable of disseminating that disease agent but shows no sign of clinical disease. So the carrier has no symptoms of a disease but still passes it on to someone else. Three types of carrier state are recognised:
· The true carrier, which is an infected individual capable of disseminating the infectious agent but which never exhibits clinical signs of disease.
· The incubatory carrier, which is an infected individual capable of disseminating the infectious agent while the disease is still in the incubatory stage.
· The convalescent carrier, which is an individual that continues to disseminate the infectious agent after the clinical signs of the disease have disappeared.