How does the mosquito act as a vector?

In medical terms, a vector is an intermediary organism that carries and transmits a disease from one host to another, usually without contracting the disease itself. When female mosquitoes feed on the blood of an animal infected with a disease (e.g. malaria, equine encephalitis), and then bites another animal, the mosquito's saliva/anticoagulant fluid infects the second animal, or human, with the disease. (In malaria, the actual fertilization of the infective Plasmodium sporozoite takes place within the mosquito.)

* In the special case of the human botfly, the mosquito has been seized by an egg-laying fly, which plants its eggs on the mosquito, and the mosquito (unharmed) carries the eggs of the fly to new human hosts.