Parasite

Definition

An organism that lives on or inside another organism (the host), deriving nutritional or other benefits while causing harm to the host, typically without immediately killing it. Parasites exhibit structural and physiological adaptations for attachment, feeding, and evasion of host defenses. The relationship is generally prolonged and intimate, distinguishing parasitism from predation or scavenging. In arthropods, parasitic strategies include ectoparasitism (living externally, as in fleas, lice, and ticks) and endoparasitism (living internally, as in some parasitic wasp larvae developing within hosts). The harm inflicted ranges from nutrient depletion and tissue damage to immunosuppression and vector-borne pathogen transmission.

Etymology

From Greek parasitos, 'one who eats at the table of another' (para-, beside + sitos, food)

Example

The cat flea (Ctenocephalides felis) is an obligate hematophagous ectoparasite that feeds repeatedly on mammalian blood, causing irritation, allergic dermatitis, and serving as a vector for Bartonella bacteria.

Related Terms

• host
• ectoparasite
• endoparasite
• parasitoid
• Parasitism
• Kleptoparasite
• Brood parasitism
• vector
• hematophagy

Usage Notes

Contrasts with parasitoid, which ultimately kills the host; with predator, which typically kills prey immediately; and with commensal, which derives benefit without harming the host. The term implies relative body size (parasite generally smaller than host) and metabolic dependence. Some authors restrict 'parasite' to cases where the host remains alive for an extended period, excluding organisms like parasitoid wasps that kill hosts relatively quickly. Microparasites (viruses, bacteria, protozoa) and macroparasites (helminths, arthropods) are distinguished in disease ecology. In entomology, 'parasite' sometimes colloquially includes parasitoids, though specialists prefer precision.