Space Leeches were green, flying creatures which attached themselves to people's heads and sucked the life out of them. They had six main appendages: two long frontal claws, two long limbs for grasping their prey and two shorter limbs at their back end. Their fleshy mouths were sucker-shaped funnels with razor-sharp teeth.
They travelled in space. When a spacecraft crashed in London on Earth in April 2010, Space Leeches swarmed from the wreck and attached themselves to the heads of Londoners, sucking the life out of them. They were susceptible to a highly contagious virus manufactured by the Eleventh Doctor from a cold virus carried by a young lad called Stephen. Detaching themselves from the virus-carrying people, they were attracted by the Doctor's biology and followed him into his TARDIS. (COMIC: Attack of the Space Leeches!)