parasite

US /ˈper.ə.saɪt/
UK /ˈper.ə.saɪt/
"parasite" picture
1.

an organism that lives in or on an organism of another species (its host) and benefits by deriving nutrients at the other's expense

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Fleas are common parasites found on dogs and cats.
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Some fungi act as parasites on plants, causing diseases.
:
leechsymbiont
2.

a person who habitually relies on or exploits others and gives nothing in return

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He's a real parasite, always living off his parents' money.
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The community tried to get rid of the social parasites who contributed nothing.
:
freeloaderspongescroungerleechhanger-on