par·a·site ˈperəˌsīt/Submit noun an organism that lives in or on another organism (its host) and benefits by deriving nutrients at the host's expense.

In biology, parasitism is a non-mutual relationship between species, where one species, the parasite, benefits at the expense of the other, the host.

a person who habitually relies on or exploits others and gives nothing in return. synonyms: hanger-on, cadger, leech, passenger