A vector is an organism that acts as an intermediary host for a parasite. Most importantly the vector transfers the parasite to the next host. Good examples of vectors are the mosquito in transmitting malaria and ticks in transferring Lyme disease.

In the case of malaria, the mosquito becomes infected by feeding on the blood of an infected human. The malaria parasite enters the mosquito's gut and enters the gut wall where it reproduces. When the parasites leave the gut they enter the salivary glands of the mosquito and then infect another human host when the mosquito next feeds.

A photograph of the assassin bug (_Triatoma infestans_).

A photograph of the assassin bug Triatoma infestans which is the most important vector for Chagas disease.
Photograph by Centers for Disease Control and Prevention, United States Federal Government.

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