A flea (order Siphonaptera) was a parasitic Earth insect. Fleas were instrumental in the transmission of Bubonic plague.
In 2156, Major Takashi Kimura compared the voreborers that he and Hoshi Sato encountered to fleas, but Sato vehemenently disagreed. (ENT - The Romulan War novel: To Brave the Storm)
In 2267, Leonard McCoy believed the rat Mickey had fleas. (TOS novel: Mission to Horatius)
While in exile on Vulcan, StarfleetAdmiral James Kirk and McCoy both referred to the HMS Bounty as a fleatrap. (TOS movie: The Voyage Home)
Q, while exiled from the Q Continuum in 2366 stated that one of his options as a lifeform was a Belzoidian Flea. (TNG episode: "Deja Q")
In the 2250s of the alternate reality caused by Nero's temporal incursion to 2233, Leonard McCoy was developing a vacine for external parasites including siphonaptera. (Star Trek (App))