parasite

Short definition from Wiktionary

Noun

1. 1
   An organism that lives on or in another organism of a different species, deriving benefit from living on or in that other organism, while not contributing towards that other organism sufficiently to cover the cost to that other organism.
2. 2
   An organism that lives on or in another organism of a different species, deriving benefit from living on or in that other organism, while not contributing towards that other organism sufficiently to cover the cost to that other organism: An internal parasite that is an animal or protist.
3. 3
   A person who lives on other people's efforts or expense and gives little or nothing back.
4. 4
   A sycophant or hanger-on.
5. 5
   A climbing plant which is supported by a wall, trellis etc.
6. 6
   A retainer or companion of an ancient Celtic warrior, who praised him in song or poetry at gatherings; a bard.
7. 7
   A component of a composite aircraft which is carried aloft and air-launched by a larger carrier aircraft or mother ship to support the primary mission of the carrier; a parasite aircraft.
8. 8
   An isolated (FSDU) (freestanding display unit )

Verb

1. 1
   To parasitize.