vector

A mosquito shortly after obtaining blood from a human (note the droplet of blood plasma being expelled as the mosquito squeezes out excess water). Mosquitos are a vector for several diseases, including viral malaria.

In epidemiology, a disease vector is any living agent that carries and transmits an infectious pathogen such as a parasite or microbe, to another living organism. Many familiar vectors, such as mosquitos, ticks, and certain flies rely on blood-feeding and can acquire or pass on pathogens during that process. Disease vectors remain a major global health challenge. The World Health Organization reports that these illnesses make up over 17% of all infectious diseases worldwide, and are responsible for hundreds of thousands of deaths every year. The term is also used for some insects and other organisms that transmit plant pathogens to crops.