Macrobe

Macrobes were a type of sentient virus. They had gaseous green bodies, big red eyes and large mouths. One group of Macrobes infiltrated the computers of the University of Rago Rago 56 Rago Reading Dome and waited for the chance to infect living brains. When the Tenth Doctor and Rose Tyler arrived, they recognised the Doctor as a Time Lord and planned to infect him, which would allow them to spread their infection throughout Time and Space. The Macrobes possessed the library's visitors through the mindlink devices used to directly send information to their wearers, but the Doctor downloaded boring books to their minds, putting them to sleep and forcing the Macrobes back into the library network, which the Doctor then disconnected, trapping them inside. Kriz then planned to have the library's technical team remove the Macrobe infection. (COMIC: The Macrobe Menace)

Behind the scenes
"Macrobe" is the fictional term for a demon used in the novel That Hideous Strength by the Irish science fiction and fantasy novelist C. S. Lewis.