Organism that lives on or in another host organism without benefiting the host – fungus is a parasite.
Merriam-Webster Online Dictionary
parasite (noun)1.
a person who exploits the hospitality of the rich and earns welcome by flattery
2.
an organism living in, with, or on another organism in - parasitism
3.
something that resembles a biological parasite in dependence on something else for existence or support without making a useful or adequate return
Merriam-Webster Online Thesaurus
parasite (noun)a person who is supported by or seeks support from another without making an adequate return
SYNONYMS:
bloodsucker, freeloader, free rider, hanger-on, moocher, parasite, sponge, spongerRELATED WORDS:
dependent; deadbeat, idler; flunky ( flunkey flunkie), henchman, lackey, satellite, stooge, sycophant, toady, yes-man; cheapskate, miser, niggard, piker, scrooge, skinflint, tightwadNEAR ANTONYMS:
benefactor, philanthropist, supporterFreeloader, mooch, hanger-on, leech, sponger
