An animal that is the carrier of a particular infectious disease (see INFECTION). A vector picks up the infectious agent (bacterium – see BACTERIA, RICKETTSIA or VIRUS) from an infected person's blood or faeces and carries it in or on its body before depositing the agent on or into a new host. Fleas, lice, mosquitoes and ticks are among common vectors of disease to humans. When a vector is used by the infectious agent to complete part of its life-cycle – for example, the malarial agent PLASMODIUM conducts part of its life-cycle in the mosquito – the vector is described as biological. If the vector simply carries the agent but is not a host for part of its life-cycle, the vector is described as mechanical. Flies, for example, may carry an infection such as bacterial dysentery from infected faeces to the food of another ‘host’.