What is parasitism in biology?

Parasitism is a relationship between different species of plants or animals where one species benefits at the expense of the other. The parasite benefits, while the host is harmed but not always killed. Parasites can be ectoparasites, like ticks and fleas, that live on the host's surface, or endoparasites, like bacteria and viruses, that live inside the host's body. Some parasites use a third organism, called a vector, to transmit them to the host. Malaria, transmitted by mosquitoes, is an example of this. Other forms of parasitism include brood parasitism, where birds lay their eggs in other species' nests, and social parasitism, where one ant species depends on the labor of another.

How does a parasite differ from a host?

In parasitism , a parasite benefits at the expense of a host, sometimes without killing it.

Can you give examples of common parasites?

Parasites live off (or in) a host and get their food from it. They occur in animals and plants. Common examples include protozoans, helminths, and ectoparasites. Protozoans like Entamoeba histolytica cause amebic dysentery through contaminated food/water. Flagellated protozoans such as Trypanosoma brucei cause sleeping sickness and are transmitted by infected tsetse flies. Helminths include roundworms, like Wuchereria bancrofti , which causes filariasis and is transmitted by arthropods. Flatworms, such as tapeworms, attach to the intestinal wall. Fleas, lice, mites, and ticks are examples of ectoparasites. Parasitic nematodes attack plant roots, bulbs, stems, leaves, or flowers.

How do parasites affect their hosts?

Parasites affect their hosts by living on or within them and benefiting at the host's expense. Parasitic infections can cause a breakdown of the host's homeostasis, disturbing organ systems and control mechanisms. Parasites can cause various diseases in humans, ranging from mild to fatal. For example, protozoans like Plasmodium cause malaria, characterized by intermittent fever, while helminths like tapeworms can cause abdominal pain or more severe conditions like cysticercosis. Some parasitic protozoans, such as Trypanosoma brucei , evade the host's immune system by changing their surface coat. Ectoparasites like fleas and ticks can transmit disease-causing organisms.

What adaptations help parasites survive in their hosts?

Parasites have developed unique adaptations to thrive within their hosts. They can alter their surface coats to evade the host's immune system , a process known as antigenic variation. Parasites may also reproduce in high numbers to increase the probability of finding new hosts. Some parasites have evolved to manipulate their hosts' behavior to increase transmission rates. For example, the sporocyst of Leucochloridium macrostomum causes the host snail's eyestalks to pulsate and become conspicuous, attracting birds that consume the eyestalks, which allows the parasite to complete its life cycle . Parasites also exhibit morphological adaptations such as hooks or suckers for attachment to the host and cyst formation to survive harsh environmental conditions and facilitate dispersal.

How is parasitism different from other relationships like mutualism or commensalism?

Symbiosis is any kind of relationship between different species that live together. These relationships can be positive, negative, or neutral for the species involved. Parasitism is an antagonistic relationship where one species, the parasite, benefits by living on or in another species, the host, which is harmed. Mutualism is a cooperative relationship where both species benefit. For example, termites and intestinal flagellated protozoans exhibit mutualism, where protozoans help termites digest wood, and neither can survive without the other. Commensalism is a relationship where one species benefits, and the other is not significantly affected. An example is the remora that attaches to sharks for transportation and feeds on the shark's leftovers without harming the shark.

parasitism

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Diagram of the life cycle of the black-legged tick (Ixodes scapularis), showing seasonal stages from engorged female laying eggs in spring, larva and nymph stages feeding on animal and human hosts in summer, and adult ticks on hosts in fall and winter. Arrows indicate progression through each stage, with illustrations of ticks and typical hosts such as deer, horses, dogs, and humans.

Two-part illustration showing parasitoid wasps. On the left, a female Ichneumonidae wasp uses her ovipositor to lay eggs inside a host larva beneath tree bark. On the right, the life cycle of a Trichogramma wasp is depicted, with a female laying eggs in a host egg, development from larva to pupa, and an adult wasp emerging.

At a Glance

parasitism summary

Quick Facts

Key People:: Rudolf Virchow; Ōmura Satoshi; William Campbell; Theobald Smith; Sir Patrick Manson

Related Topics:: kleptoparasitism; endoparasitism; social parasitism; hyperparasitism; facultative parasite

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dodder Dodder (*Cuscuta*), a seed-producing parasite, entwined around blueberry (*Vaccinium*).

parasitism

biology

Also known as: parasite

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Oct. 14, 2026 •History

Top Questions

What is parasitism in biology?
Parasitism is a relationship between different species of plants or animals where one species benefits at the expense of the other. The parasite benefits, while the host is harmed but not always killed.

Parasites can be ectoparasites, like ticks and fleas, that live on the host's surface, or endoparasites, like bacteria and viruses, that live inside the host's body. Some parasites use a third organism, called a vector, to transmit them to the host. Malaria, transmitted by mosquitoes, is an example of this.

Other forms of parasitism include brood parasitism, where birds lay their eggs in other species' nests, and social parasitism, where one ant species depends on the labor of another.
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How does a parasite differ from a host?
In parasitism, a parasite benefits at the expense of a host, sometimes without killing it.
- Parasite A parasite lives on or in another organism, the host, to obtain nutrients, support, or other resources. Parasites are typically smaller than their hosts and often harm the host. They can be ectoparasites (like ticks and fleas) that live on the host's surface or endoparasites (like bacteria and viruses) that live inside the host.
- Host The host is the organism that supports a parasite. The host is often harmed in the relationship.
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Can you give examples of common parasites?
Parasites live off (or in) a host and get their food from it. They occur in animals and plants. Common examples include protozoans, helminths, and ectoparasites.

Protozoans like Entamoeba histolytica cause amebic dysentery through contaminated food/water. Flagellated protozoans such as Trypanosoma brucei cause sleeping sickness and are transmitted by infected tsetse flies. Helminths include roundworms, like Wuchereria bancrofti, which causes filariasis and is transmitted by arthropods. Flatworms, such as tapeworms, attach to the intestinal wall. Fleas, lice, mites, and ticks are examples of ectoparasites.

Parasitic nematodes attack plant roots, bulbs, stems, leaves, or flowers.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do parasites affect their hosts?
Parasites affect their hosts by living on or within them and benefiting at the host's expense. Parasitic infections can cause a breakdown of the host's homeostasis, disturbing organ systems and control mechanisms.

Parasites can cause various diseases in humans, ranging from mild to fatal. For example, protozoans like Plasmodium cause malaria, characterized by intermittent fever, while helminths like tapeworms can cause abdominal pain or more severe conditions like cysticercosis. Some parasitic protozoans, such as Trypanosoma brucei, evade the host's immune system by changing their surface coat. Ectoparasites like fleas and ticks can transmit disease-causing organisms.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What adaptations help parasites survive in their hosts?
Parasites have developed unique adaptations to thrive within their hosts. They can alter their surface coats to evade the host's immune system, a process known as antigenic variation. Parasites may also reproduce in high numbers to increase the probability of finding new hosts.

Some parasites have evolved to manipulate their hosts' behavior to increase transmission rates. For example, the sporocyst of Leucochloridium macrostomum causes the host snail's eyestalks to pulsate and become conspicuous, attracting birds that consume the eyestalks, which allows the parasite to complete its life cycle.

Parasites also exhibit morphological adaptations such as hooks or suckers for attachment to the host and cyst formation to survive harsh environmental conditions and facilitate dispersal.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How is parasitism different from other relationships like mutualism or commensalism?
Symbiosis is any kind of relationship between different species that live together. These relationships can be positive, negative, or neutral for the species involved.
- Parasitism is an antagonistic relationship where one species, the parasite, benefits by living on or in another species, the host, which is harmed.
- Mutualism is a cooperative relationship where both species benefit. For example, termites and intestinal flagellated protozoans exhibit mutualism, where protozoans help termites digest wood, and neither can survive without the other.
- Commensalism is a relationship where one species benefits, and the other is not significantly affected. An example is the remora that attaches to sharks for transportation and feeds on the shark's leftovers without harming the shark.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

parasitism, relationship between two species of plants or animals in which one benefits at the expense of the other, sometimes without killing the host organism.

Parasites may be characterized as ectoparasites—including ticks, fleas, leeches, and lice—which live on the body surface of the host and do not themselves commonly cause disease in the host; or endoparasites, which may be either intercellular (inhabiting spaces in the host’s body) or intracellular (inhabiting cells in the host’s body). Intracellular parasites—such as bacteria or viruses—often rely on a third organism, known as the carrier, or vector, to transmit them to the host. Malaria, which is caused by a protozoan of the genus Plasmodium transmitted to humans by the bite of an anopheline mosquito, is an example of this interaction. The plant ailment known as Dutch elm disease (caused by the fungus Ceratocystis ulmi) can be spread by the European elm bark beetle.

A different form of parasitism called brood parasitism is practiced by most species of cuckoos and all cowbirds. Those birds do not build nests of their own but deposit their eggs in the nests of other species and abandon them there, with the hope that adult birds of other species will raise the abandoned young as their own. The cowbird’s parasitism does not necessarily harm the host or the host’s brood; however, the cuckoo may remove one or more host eggs to reduce the suspicion surrounding the presence of its egg, and the young cuckoo may heave the host’s eggs and nestlings from the nest.

parasitism

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