Virus

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses infect all types of lifeforms (both carbon and noncarbon based, from animals and plants to microorganisms, including bacteria and archaea.

Viruses spread in many ways; viruses in plants are often transmitted from plant to plant by insects that feed on plant sap; viruses in animals can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. The range of host cells that a virus can infect is called its "host range". This can be narrow or, as when a virus is capable of infecting many species, broad.