parasite
n//ˈpæɹəˌsaɪt//parasit
(biology) 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.
(pejorative) A person who lives on other people's efforts or expense and gives little or nothing back. [from 16th c.]