A vector is an agent, such as a virus (typically a retrovirus) or a small piece of DNA called a plasmid, that carries a modified or foreign gene. When used in gene therapy, a vector delivers the desired gene to a target cell. Once inside the host, the vector can replicate and thus produce clones (exact copies) of the gene.

A vector is also an animal, usually an insect or a tick, that transmits parasitic microorganisms – and therefore the diseases they cause – from person to person or from infected animals to human beings. Mosquitoes, for example, are vectors of malaria, filariasis, and yellow fever.