parasite

noun

par·​a·​site ˈper-ə-ˌsīt 

ˈpa-rə-

plural parasites

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 1 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

Synonyms

• leech
• sponge
• freeloader
• dependent
• sponger
• moocher
• henchman
• hanger-on
• bloodsucker

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.

Recent Examples on the Web

The United States is dropping about 100 million sterile flies each week in southern Mexico to help slow the spread of the parasite. —Clara Migoya, AZCentral.com, 16 July 2025

Colton was also prescribed a parasite treatment due to diarrhea, the warrant affidavit said. —Staff Report, Hartford Courant, 16 July 2025

Scale populations are normally kept in check by environmental factors and natural enemies such as parasites and predators. —Tim Johnson, Chicago Tribune, 13 July 2025

This is because in Asia, there are natural predators and parasites that keep the box tree moth population in control. —Chris McKeown, The Enquirer, 12 July 2025

Word History

Etymology

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

First Known Use

1539, in the meaning defined at sense 2

Rhymes for parasite

• acolyte
• aconite
• allanite
• alunite
• amberlite
• ammonite
• amorite
• amphitrite
• analyte
• anchorite
• andesite
• anhydrite

Kids Definition

parasite

noun

par·​a·​site ˈpar-ə-ˌsīt 

1

: a person who lives at the expense of another

2

: a living thing which lives in or on another living thing in parasitism

3

: a person dependent on something else for life or support without making proper return

Medical Definition

parasite

noun

par·​a·​site ˈpar-ə-ˌsīt 

: an organism living in, with, or on another organism in order to obtain nutrients, grow, or multiply often in a state that directly or indirectly harms the host (see host sense 1)

The blood schizogonic cycle of human malaria parasites has thus far been the most exhaustively studied phase of parasite development.

—Dominique Mazier et al., Nature

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.

These cells call up a portion of the immune system that identifies and destroys cells infected with bacteria, viruses, and parasites.

—L. Seachrist, Science News

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, Scientific American

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., Science