Dord

The Dord were a species of brain parasite that transported themselves through language. In 1939, Earth was invaded by the Dord, but their invasion was limited to a single dictionary. (PROSE: Time Traveller's Diary)

Behind the scenes
This is a reference to a real-life error in the 1934 edition of the New International Dictionary wherein an editor misinterpreted "D or d", an abbreviation for "density" as "dord". The error was not discovered until 1939.