par·​a·​site | \ ˈper-ə-ˌsīt How to pronounce parasite (audio) , ˈpa-rə- \
plural parasites

Definition of parasite

1 : an organism living in, on, or with another organism in order to obtain nutrients, grow, or multiply often in a state that directly or indirectly harms the host (see host entry 3 sense 2a) Now the death of its host is certainly a setback to any parasite. To some (like the tapeworm) it is fatal; but smarter ones (like the louse) simply go off in search of a new host.— David Jones The fungus is an obligate parasite, that is, it must have a living host (tobacco) on which to grow and complete its life cycle.— G. B. Lucas The blood schizogonic cycle of human malaria parasites has thus far been the most exhaustively studied phase of parasite development.— Dominique Mazier et al. Sadly, the vireo is vulnerable to a nest parasite, the brown-headed cowbird … . The cowbird lays its much-larger eggs in the vireo's nest, which hatch first and place such a high food demand on its tiny "parents" that the vireo young go unfed.— Karen D. Fishler

Note: Some restrict the use of parasite to include only multicellular forms (such as protozoans and helminths) while others use it to include bacteria and viruses.

Unlike bacteria or viruses, parasites undergo a metamorphosis during their life cycles that presents the human immune system with a moving target.— Lawrence M. Fisher Like all viruses, the human immunodeficiency virus (HIV) is an intracellular parasite: the virus particle itself is inert and cannot propagate or do any damage until it enters a host cell.— Jonathan N. Weber and Robin A. Weiss Salmonella species are intracellular parasites, and it is thought that these bacteria gain access to their host by penetrating through intestinal epithelial cells.— B. Brett Finlay et al. — see also ectoparasite, endoparasite
2 : someone or something that resembles a biological parasite in living off of, being dependent on, or exploiting another while giving little or nothing in return But the frequent and familiar companions of the great, are those parasites, who practise the most useful of all arts, the art of flattery …— Edward Gibbon Their lyrics … convey a bilious contempt for the city's wealthy parasites— Philip Montoro In their view, the country is afflicted with a class of parasites—"Career politicians," who devote their lives to perpetuating themselves in office by spending the people's money.— Hendrick Hertzberg Regulatory agencies have stripped Holyfield of his boxing license now, protecting him from his pride and from the parasites who can still squeeze money out of the faded neon in his name.— Dan Le Batard

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Choose the Right Synonym for parasite

parasite, sycophant, toady, leech, sponge mean a usually obsequious flatterer or self-seeker. parasite applies to one who clings to a person of wealth, power, or influence or is useless to society. a jet-setter with an entourage of parasites sycophant adds to this a strong suggestion of fawning, flattery, or adulation. a powerful prince surrounded by sycophants toady emphasizes the servility and snobbery of the self-seeker. cultivated leaders of society and became their toady leech stresses persistence in clinging to or bleeding another for one's own advantage. a leech living off his family and friends sponge stresses the parasitic laziness, dependence, and opportunism of the cadger. a shiftless sponge, always looking for a handout

Examples of parasite in a Sentence

Many diseases are caused by parasites. She's a parasite who only stays with him for the money. These new companies are parasites feeding off the success of those who spent the last decade establishing the industry.
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Recent Examples on the Web Over time, populations of prickly pears resistant to the parasite evolved in Australia, and this is a first, though not fatal, complication that will, however, require a more careful control of the cactus population in the future. Longreads, "Why Bumblebees Love Cats and Other Beautiful Relationships," 23 Mar. 2021 But hamsters, which are a model organism widely used in laboratory experiments, are not a natural host of the parasite. Jim Daley, Smithsonian Magazine, "Dogs Infected With a Deadly Human Parasite Smell Better to Insect Vectors," 18 Mar. 2021 Treating malaria patients is another important prevention tactic; mosquitoes can’t pass on the parasite from people who no longer have it. Popular Science, "COVID-19 could make malaria surge, but African aid workers are fighting back," 7 Jan. 2021 Photographer Anand Varma has spent years capturing the world of mind-controlling parasites such as this one, which will use its powers to widen the crab’s abdomen, creating a womb for the parasite to fill with its own eggs. Nick Nichols, Photography, "Nat Geo’s 21 most compelling images of the 21st century," 8 Dec. 2020 Then, in 1926, a solution was finally found: an Argentine lepidopteran (moth) known as Cactoblastis cactorum, a parasite of various species of Opuntia. Longreads, "Why Bumblebees Love Cats and Other Beautiful Relationships," 23 Mar. 2021 Efforts are already under way, for example, for an mRNA vaccine for malaria—a parasite that each year kills hundreds of thousands of people, mostly children, and is notoriously hard to vaccinate against. Zeynep Tufekci, The Atlantic, "3 Ways the Pandemic Has Made the World Better," 18 Mar. 2021 However, all of the 64 parasite-free sea slugs stayed intact. Matt Simon, Wired, "Hey, So These Sea Slugs Decapitate Themselves and Grow New Bodies," 8 Mar. 2021 Quinine wasn't toxic to the flu virus since the infective agent that caused flu — a virus — differed from the infective agent that induces malaria — a parasite. Kristen Rogers, CNN, "Bloodletting and gas fumes: Quack treatments of the 1918 flu," 17 Oct. 2020

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'parasite.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of parasite

1539, in the meaning defined at sense 1

History and Etymology for parasite

Middle French, from Latin parasitus, from Greek parasitos, from para- + sitos grain, food

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Time Traveler for parasite

Time Traveler

The first known use of parasite was in 1539

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Statistics for parasite

Last Updated

20 Apr 2021

Cite this Entry

“Parasite.” Dictionary, Merriam-Webster, Accessed 22 Apr. 2021.

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More Definitions for parasite



English Language Learners Definition of parasite

: an animal or plant that lives in or on another animal or plant and gets food or protection from it
disapproving : a person or thing that takes something from someone or something else and does not do anything to earn it or deserve it


par·​a·​site | \ ˈper-ə-ˌsīt How to pronounce parasite (audio) \

Kids Definition of parasite

1 : a living thing (as a flea, worm, or fungus) that lives in or on another living thing and gets food and sometimes shelter from it and usually causes harm to it
2 : a person who lives at the expense of another


par·​a·​site | \ ˈpar-ə-ˌsīt How to pronounce parasite (audio) \

Medical Definition of parasite

: an organism living in, with, or on another organism in parasitism

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