Virus

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea.

Viruses turn up in practically every genre of television fiction and can range from mundane illnesses such as the common cold to fictional ailments such as the vampire virus from season three of Being Human or the Clendian flu from the "That Old Black Magic" episode of Farscape.