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 "D or d", an abbreviation for "density", was misfiled as a word, "dord". The error was not discovered until 1939.