A vector in biology is an animal on or in which a small living thing gets transported. The vector gets no benefit and sometimes loses fitness by the arrangement.
The term is most used for the transport of parasites and agents of infection or disease. So, deadly diseases, such as malaria and dengue fever, are carried by some mosquitoes.
The study of vectors gives us knowledge about the life cycle of parasitic diseases, and this helps us control those diseases.
Examples
Insects
Flies
Mosquitoes
Mosquitoes of the Anopheles genus transmit human malaria and elephantiasis (a roundworm disease). Aedes mosquitoes are vectors of avian malaria, dengue fever, yellow fever and chikungunya (a viral disease). Sand flies transmit leishmaniasis, bartonellosis and pappataci fever. Tsetse flies Several genera are vectors of human African trypanosomiasis also known as "African sleeping sickness".
Bugs
Aphids are the vectors of many viral diseases in plants. Triatomine bugs such as Rhodnius prolixus are vectors of Chagas disease.
Other insects
Fleas such as the human flea, Pulex irritans and the Oriental rat flea, Xenopsylla cheopis, transmit bubonic plague, murine typhus and tapeworms. Rodents, such as rats and mice carry the fleas, and spread them. Glassy-winged sharpshooter (a leafhopper) transmits the Xylella fastidiosa bacterium among plants, resulting in diseases of grapes, almonds, and many other cultivated plants.
Other groups
Ticks of the genus Ixodes are vectors of Lyme disease and babesiosis. and along with lice transmit various members of the bacterial genus Rickettsia. Cyclopoid copepods: a number of species transmit the nematode Dracunculus medinensis. Mammals Bats which represent about 20% of all known mammalian species act as both a natural reservoirs for viruses such as the Hendra virus (HeV) and the SARS like coronaviruses and in many cases as a vector for various viruses such as the lyssaviruses including the rabies virus. Felids (cats) are the primary hosts for Toxoplasma gondii a parasitic protozoan which causes Toxoplasmosis. Approximately 30% of the human population is infected with Toxoplasmosis.