Noun

(en noun)

(pejorative) A person who lives on other people's efforts or expense and gives little or nothing back.

(biology) an organism that lives on or in another organism, 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.

* {{quote-magazine, date=2013-03

, author= , title=The Smallest Cell , volume=101, issue=2, page=83 , magazine= citation , passage=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.}}

''Lice, fleas, ticks and mites are widely spread parasites .

(literary, poetic) A climbing plant which is supported by a wall, trellis etc.

* 1813 , (Percy Bysshe Shelley), Queen Mab , I:

Her golden tresses shade / The bosom’s stainless pride, / Curling like tendrils of the parasite / Around a marble column.

Noun

(en-noun) (wikipedia virus) (Virus)

(archaic) Venom, as produced by a poisonous animal etc.

(pathology, microbiology, virology) A submicroscopic, non-cellular structure consisting of a core of DNA or RNA surrounded by a protein coat, that requires a living host cell to replicate, and often causes disease in the host organism.

* {{quote-magazine, year=2013, month=May-June, author= Katie L. Burke

, title= In the News , volume=101, issue=3, page=193, magazine=(American Scientist) , passage=Bats host many high-profile viruses that can infect humans, including severe acute respiratory syndrome and Ebola.}}

* 2001 , Leslie Iversen, Drugs: A Very Short Introduction (Oxford 2001, p. 64)

Viruses are the smallest and most simplified forms of life.

A disease caused by these organisms.

(computing) A program which can covertly transmit itself between computers via networks (especially the Internet) or removable storage such as CDs, USB drives, floppy disks, etc., often causing damage to systems and data; also computer virus.