Flea

Fleas were animals native to Earth commonly found on dogs, (TV: The Two Doctors) and in "a beggar's robes". (TV: The Masque of Mandragora) According to the Eleventh Doctor, even educated fleas "do it". (TV: Hide)

Rose-the-cat is infected with fleas. (COMIC: A Rose by Any Other Name)

In the 14th century, fleas on rats carried the bubonic plague throughout Europe. Terileptil prisoners who found themselves near London in 1666 exploited this to engineer a new plague variant that was injected into the rats. This plague was prevented by the Fifth Doctor. (TV: The Visitation)

Behind the scenes

The Doctor's mention of the species in Hide is in reference to Ella Fitzgerald's "Let's Do it, Let's Fall in Love":

Birds do it, bees do it. Even educated fleas do it. Let's do it. Let's fall in love.