Parasitism: Definition and Examples

What Are Parasites and Why Do We Need Them?

The wood tick is an example of an ectoparasite.
ArtBoyMB / Getty Images

Parasitism is defined as a relationship between two species in which one organism (parasite) lives on or within the other organism (host), causing the host some degree of harm. A parasite reduces its host's fitness but increases its own fitness, usually by gaining food and shelter.

Key Takeaways: Parasitism

  • Parasitism is a type of symbiotic relationship in which one organism benefits at the expense of another.
  • The species that benefits is called the parasite, while the one that is harmed is called the host.
  • Over half of all known species are parasites. Parasites are found in all biological kingdoms.
  • Examples of human parasites include roundworms, leeches, ticks, lice, and mites.

The term "parasite" comes from the Greek word parasitos, which means "one who eats at the table of another." The study of parasites and parasitism is called parasitology.

There are parasites belonging to every biological kingdom (animals, plants, fungi, protozoa, bacteria, viruses). In the animal kingdom, every parasite has a free-living counterpart. Examples of parasites include mosquitoes, mistletoe, roundworms, all viruses, ticks, and the protozoan that causes malaria.

Parasitism vs. Predation

Both parasites and predators rely on another organism for one or more resources, but they have numerous differences. Predators kill their prey in order to consume it. As a result, predators tend to be physically larger and/or stronger than their prey. Parasites, on the other hand, tend to be much smaller than their host and do not normally kill the host. Instead, a parasite lives on or within the host for a period of time. Parasites also tend to reproduce much more quickly than hosts, which is not usually the case in predator-prey relationships.

Parasitism vs. Mutualism vs. Commensalism

Parasitism, mutualism, and commensalism are three types of symbiotic relationships between organisms. In parasitism, one species benefits at the expense of the other. In mutualism, both species benefit from the interaction. In commensalism, one species benefits, while the other is neither harmed nor helped.

Types of Parasitism

There are multiple ways to classify types of parasitism.

Parasites may be grouped according to where they live. Ectoparasites, such as fleas and ticks, live on the surface of a host. Endoparasites, such as intestinal worms and protozoa in blood, live inside a host's body. Mesoparasites, such as some copepods, enter the opening of a host body and partially embed themselves.

The human head louse is a directly-transmitted obligate ectoparasite.
The human head louse is a directly-transmitted obligate ectoparasite. SCIEPRO / Getty Images

The life cycle can be a basis for classifying parasites. An obligate parasite requires a host in order to complete its life cycle. A facultative parasite can complete its life cycle without a host. Sometimes location and life cycle requirements may be combined. For example, there are obligate intracellular parasites and facultative intestinal parasites.

Parasites may be classified according to their strategy. There are six major parasite strategies. Three relate to parasite transmission:

  • Directly transmitted parasites, such as fleas and mites, reach their host on their own.
  • Trophically transmitted parasites, such as trematodes and roundworms, are eaten by their host.
  • Vector transmitted parasites rely on an intermediate host to transport them to their definitive host. An example of a vector transmitted parasite is the protozoan that causes sleeping sickness (Trypanosoma), which is transported by biting insects.

The other three strategies involve the parasite's effect on its host:

  • Parasitic castrators either partly or fully inhibit a host's reproductive ability but allow the organism to live. The energy the host would have put toward reproduction is diverted toward supporting the parasite. An example is the barnacle Sacculina, which degenerates the gonads of crabs such that males develop the appearance of females.
  • Parasitoids eventually kill their hosts, making them nearly predators. All examples of parasitoids are insects that lay their eggs on or inside the host. When the egg hatches, the developing juvenile serves as food and shelter.
  • A micropredator attacks more than one host so that most host organisms survive. Examples of micropredators include vampire bats, lampreys, fleas, leeches, and ticks.

Other types of parasitism include brood parasitism, where a host raises the young of the parasite (e.g., cuckoos); kleptoparasitism, in which a parasite steals the host's food (e.g., skuas stealing food from other birds); and sexual parasitism, in which males rely on females for survival (e.g., anglerfish).

The banded caterpillar parasite wasp uses its long ovipositor to lay eggs inside its host.
The banded caterpillar parasite wasp uses its long ovipositor to lay eggs inside its host. Louise Docker Sydney Australia / Getty Images

Why We Need Parasites

Parasites harm their hosts, so it's tempting to think they should be eradicated. Yet, at least half of all known species are parasitic. Parasites serve an important role in an ecosystem. They help control dominant species, allowing for competition and diversity. Parasites transfer genetic material between species, serving a role in evolution. In general, the presence of parasites is a positive indication of ecosystem health.


  • ASP (Australian Society of Parasitology Inc.) and the ARC/NHMRC (Australian Research Council/National Health and Medical Research Council) Research Network for Parasitology (2010). "Overview of Parasitology". ISBN 978-1-8649999-1-4.
  • Combes, Claude (2005). The Art of Being a Parasite. The University of Chicago Press. ISBN 978-0-226-11438-5.
  • Godfrey, Stephanie S. (2013). "Networks and the Ecology of Parasite Transmission: A Framework for Wildlife Parasitology". Wildlife. 2: 235–245. doi:10.1016/j.ijppaw.2013.09.001
  • Poulin, Robert (2007). Evolutionary Ecology of Parasites. Princeton University Press. ISBN 978-0-691-12085-0.
mla apa chicago
Your Citation
Helmenstine, Anne Marie, Ph.D. "Parasitism: Definition and Examples." ThoughtCo, Sep. 3, 2021, Helmenstine, Anne Marie, Ph.D. (2021, September 3). Parasitism: Definition and Examples. Retrieved from Helmenstine, Anne Marie, Ph.D. "Parasitism: Definition and Examples." ThoughtCo. (accessed June 4, 2023).