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)

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.