parasite

1. (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.Examples: "Lice, fleas, ticks, and mites are widely spread parasites."; "...and the mistletoe crept round many of the oaks—that pleasant parasite, whose associations belong rather to the hearth and lighted hall than to its native branches."; "It is likely that the long evolutionary trajectory of Mycoplasma went from a reductive autotroph to oxidative heterotroph to a cell-wall–defective degenerate parasite. This evolutionary trajectory assumes the simplicity to complexity route of biogenesis, a point of view that is not universally accepted."

2. (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.Examples: "He lacks, however, even the basic Dr. Kildare lingo about medical issues. For instance, he regularly misidentifies poppers as amyl nitrate, instead of amyl nitrite — two forms of heart medicine with different affects ^([sic]). Salmonella, a bacteria, is not usually classified as a parasite. And he mistakenly writes, "lenti means slow" in the lentiviruses. ("Lenti" means lens-shaped, biologists classify by structure, not by behavior.)"; "Select a disease class Virus: Fastest evolution, Most affected by environment, Bonus to infectivity Bacteria: Medium evolution, Normally affected by environment, Bonus to drug resistance Parasite: Slowest evolution, Least affected by environment, Low visibility"; "Any germs or parasites on the surface of the meat that you could conceivably wash off are gonna die as soon as you cook it, so don't bother washing it."informalspecifically

3. (derogatory) A person who lives on other people's efforts or expense and gives little or nothing back.Examples: "A royalist who publicly called for a military intervention to protect the parasite monarchy."; "Of all the corrupting effects of wealth there is none worse than this, that it makes the wealthy (and their parasites) think in some way divine, or at least a lovely character of the mind, what is in truth nothing but their power of luxurious living."Synonyms: bloodsucker, bummer, freeloader, free rider, gadfly, leech, mooch, moocherderogatory

4. (derogatory) A sycophant or hanger-on.derogatory

5. (literary, poetic) A climbing plant which is supported by a wall, trellis etc.Examples: "Her golden tresses shade / The bosom's stainless pride, / Curling like tendrils of the parasite / Around a marble column."literarypoetic

6. (historical) A retainer or companion of an ancient Celtic warrior, who praised him in song or poetry at gatherings; a bard.historical