Solution:

Epidemiological triad consists of an external agent, a host and an environment in which host and agent are brought together, causing the disease to occur in the host. A vector, an organism which transmits infection by conveying the pathogen from one host to another without causing disease itself, may be part of the infectious process.

A classic example of a vector is the Anopheles mosquito. As the mosquito ingests blood from an infected host, it picks up the parasite plasmodium.