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Parasitism – Definition, Types, Examples

What is Parasitism?

  • Parasitism represents a unique form of symbiotic interaction, characterized by a prolonged association between two distinct species. Within this relationship, one organism, termed the parasite, derives benefits at the detriment of the other, known as the host.
  • The etymological roots of the term “parasite” trace back to the Greek word παράσιτος (parasitos), which translates to “one who eats at the table of another,” aptly capturing the essence of the relationship. Scientifically, parasitism is delineated as an interspecies relationship wherein the parasite resides either on or within the host organism, reaping benefits while inflicting harm upon the host.
  • The organism that accrues advantages is classified as the parasite, while the adversely affected entity is designated as the host. Notable instances of parasitic entities encompass tapeworms, fleas, and barnacles, among others. For instance, tapeworms, a category of flatworms, colonize the intestines of various animals, consuming partially digested food and depriving the host of essential nutrients.
  • From a structural and functional perspective, parasites exhibit remarkable adaptations that facilitate their parasitic mode of life. As elucidated by entomologist E. O. Wilson, parasites can be perceived as “predators that consume prey in units of less than one.”
  • The realm of parasites is vast, encompassing single-celled protozoans, multicellular animals, fungi, and even certain plants. Parasitic strategies can be broadly categorized based on their mode of transmission and degree of invasiveness. Endoparasites, for instance, inhabit the internal environment of the host, while ectoparasites reside externally on the host’s surface.
  • The dynamics of parasitism bear similarities to predator-prey interactions, albeit with distinctions. Typically, parasites exhibit a smaller size relative to their hosts, maintain prolonged associations without causing immediate host death, and often demonstrate accelerated reproductive rates. The ramifications of parasitism on host fitness span a spectrum, ranging from specific pathologies, such as parasitic castration, to alterations in host behavior.
  • The evolutionary trajectory of parasitism is intricate, with some relationships evolving towards mutualism, while others retain their parasitic nature. Historically, awareness of parasites like roundworms and tapeworms dates back to ancient civilizations, including Egypt and Greece.
  • The advent of modern parasitology can be attributed to the 19th century, bolstered by pioneering observations from researchers like Antonie van Leeuwenhoek and Francesco Redi. In cultural contexts, parasitism often carries negative implications, a sentiment echoed in literary works and cinematic portrayals.
  • In conclusion, parasitism offers a fascinating lens into the complexities of interspecies interactions, underscoring the delicate balance of benefits and detriments inherent to such relationships. As research continues to unravel the intricacies of parasitism, it remains a testament to the adaptive strategies and evolutionary trajectories that shape the natural world.

Definition of Parasitism

Parasitism is a symbiotic relationship between two species in which one organism, the parasite, derives benefits at the expense of the other organism, the host, often causing it harm.

Mechanisms of Dispersal

  • Dispersal is essential for all species, regardless of whether they are free-living or parasitic, because a population confined to a small area runs the risk of extinction if environmental conditions become unfavourable, and because dispersal reduces inbreeding and the loss of evolutionary adaptability.
  • A third point is significant for parasites: dispersal may lessen the likelihood of hosts being overinfected. Important features of dispersal include dispersal over short distances away from a single host, dispersal in space and range extension over greater distances, and dispersal over time.
  • Trematode larvae demonstrate that a single stage can be responsible for all three components of dispersal. Larvae (cercariae) are frequently discharged against their will into the respiratory currents of snails in which they have matured, resulting in their dispersal away from the host.
  • They actively swim and use their tails to stay afloat, allowing them to be disseminated across great distances by water currents. In several species, the tail is equipped with specific flotation structures that extend the period of floating.
  • Adult flukes generate eggs, and larvae in their snail hosts are produced over extended periods, months, or even years, resulting in the dispersion of the parasite over time.

Types of Parasitism 

Parasitism, a multifaceted biological interaction, can be classified based on various criteria, ranging from the location of the parasite to its impact on the host. This article elucidates the diverse types of parasitism, providing a structured understanding of this intricate relationship.

1. Based on Location:

  • Ectoparasites: These parasites inhabit the external surface of the host. Common examples include fleas and ticks.
  • Endoparasites: Residing within the host’s body, endoparasites encompass entities like roundworms and certain protozoa.
  • Meso Parasites: These parasites penetrate the host through an opening and anchor themselves. Copepods serve as a representative example.

2. Life Cycle-Based Classification:

  • Obligate Parasite: These parasites are entirely reliant on their host for survival and completing their life cycle. An exemplar is the head louse, which cannot survive away from the human scalp.
  • Facultative Parasite: Possessing the ability to complete their life cycle independently of the host, these parasites can also lead a free-living existence. Strongyloides stercoralis is a notable facultative parasite.
  • Monogenic Parasites: Requiring only a singular host to complete their life cycle, monogenic parasites have a straightforward life cycle.
  • Digenetic Parasites: These parasites necessitate multiple hosts to achieve a full life cycle. Plasmodium vivax, responsible for malaria, typifies this category.

3. Strategy-Based Classification:

  • Directly Transmitted Parasites: These parasites can autonomously reach their host, with fleas being a prime example.
  • Trophically Transmitted Parasites: Acquired by hosts through consumption, roundworms and trematodes are classic representatives.
  • Vector Transmitted Parasites: Relying on an intermediary host for transmission, the protozoan causing sleeping sickness, transmitted via insect bites, exemplifies this category.

4. Impact on Host:

  • Parasitic Castrators: These parasites diminish the reproductive capability of the host, as seen with the barnacle Sacculina affecting crabs.
  • Parasitoids: Essentially carnivorous, parasitoids eventually lead to host death. They are predominantly insects depositing their eggs within the host.
  • Micropredator: Engaging with multiple hosts, micropredators include entities like vampire bats and fleas.
  • Necrotrophic and Biotrophic Parasites: While necrotrophic parasites cause host death through tissue consumption, biotrophic parasites ensure host survival for their sustenance.

5. Size-Based Classification:

  • Macroparasitism: Encompassing parasites visible to the naked eye.
  • Microparasitism: Microscopic parasites, predominantly unicellular like certain protozoa.

6. Miscellaneous Classifications:

  • Brood Parasitism: Here, young parasites are nurtured by host species, as observed in cuckoos.
  • Klepto Parasitism: This involves the theft of food, as seen in certain bird species.
  • Sexual Parasitism: Characterized by male dependence on females for survival, exemplified by anglerfish.
  • Epiparasitism: This involves a parasite targeting another parasite, such as a protozoan within a flea.
  • Social Parasitism: Observed in social insects, where intruders exploit the host’s social structure.
  • Hyperparasitism: This involves a parasite preying on another parasite, potentially leading to multiple levels of parasitism.
  • Adelphoparasitism: Occurring between closely related species, this form of parasitism is often observed within the same genus or family.

In conclusion, the realm of parasitism is vast and diverse, with each classification offering insights into the intricate relationships between parasites and their hosts. Understanding these classifications aids in comprehending the evolutionary strategies and survival mechanisms employed by parasites.

Interesting Facts on Parasitism

Parasitism, a distinct form of symbiotic interaction, is characterized by a unilateral benefit where only one organism, the parasite, gains advantage, often at the expense of the other, the host. This relationship, prevalent across the biological spectrum, offers a myriad of fascinating facets:

  1. Ubiquity of Parasitism: Parasitism is not restricted to any specific taxonomic group. Both flora and fauna can find themselves entangled in parasitic relationships, either as the parasite or the host.
  2. Diverse Infiltration: Parasites exhibit remarkable adaptability in their modes of invasion. They can colonize various components of an animal, from the circulatory system and vital organs to external surfaces. Similarly, plants are not immune, with parasites targeting their vascular systems and other integral parts.
  3. Consequences of Parasitism: The ramifications of parasitism can be severe. While some hosts may experience diminished health or compromised functions, in extreme cases, parasitism can culminate in the host’s demise.
  4. Multiplicity of Parasitic Forms: The realm of parasitism is vast, encompassing a plethora of types and manifestations. This diversity underscores the evolutionary strategies parasites have developed to ensure their survival and propagation.
  5. Inherent Nature of Symbiosis: While parasitism might seem inherently detrimental, it is essential to recognize it as a natural form of symbiosis. Just as mutualism sees both parties benefit, parasitism is a skewed balance where the advantage is one-sided.

In essence, parasitism, with its intricate dynamics and myriad manifestations, remains a focal point of study in biological sciences, shedding light on the complex interplay of organisms within ecosystems.

Evolutionary Ecology

Evolutionary ecology delves into the intricate relationships and interactions between organisms and their environments, with a particular emphasis on the evolutionary processes that shape these dynamics. One of the most captivating aspects of evolutionary ecology is the phenomenon of parasitism, which offers a window into the complex interplay of co-evolution, adaptation, and survival strategies.

  1. Prevalence of Parasitism: Parasitism is not a fringe occurrence in the natural world. In fact, almost every free-living animal is likely to host at least one parasitic species. Vertebrates alone are estimated to be hosts to a staggering 75,000 to 300,000 helminth species, alongside a myriad of parasitic microbes.
  2. Diverse Host Range: Mammals, including humans, are not the only victims of parasitic invasions. A typical mammalian species can be a host to multiple nematode, trematode, and cestode species. Humans, for instance, harbor 342 helminth species and 70 protozoan species.
  3. Ecological Significance: Parasites play a pivotal role in regulating host populations, influencing as much as three-quarters of the interactions in food webs. Remarkably, nearly 40% of all known animal species have been classified as parasitic.
  4. Fossil Evidence: While direct evidence of parasitism in the fossil record is scarce, certain indications, such as perforations in Tyrannosaurus mandibles, suggest the presence of parasites in ancient times.
  5. Coevolutionary Dynamics: The evolutionary dance between parasites and their hosts is a testament to the power of natural selection. As parasites evolve to exploit their hosts more effectively, hosts in turn evolve defenses to counteract these threats. This dynamic can lead to a range of outcomes, from heightened virulence in the parasite to the development of mutualistic relationships.
  6. Cospeciation: In some instances, the evolutionary trajectories of parasites and their hosts become so intertwined that their phylogenies mirror each other, a phenomenon known as cospeciation. This intricate relationship can offer insights into broader evolutionary patterns and connections between different taxa.
  7. Behavioral Manipulation: Some parasites possess the remarkable ability to alter the behavior of their hosts to enhance their own survival and transmission. For instance, certain flukes can manipulate the behavior of fish, making them more susceptible to predation and thus facilitating the parasite’s life cycle.
  8. Trait Evolution: Over evolutionary timescales, parasites may undergo significant changes in their traits, either losing or enhancing certain functions to optimize their parasitic strategy. For example, many insect ectoparasites have lost their ability to fly, relying instead on their hosts for mobility.
  9. Host Defenses: Host organisms have evolved a plethora of defenses against parasitic invasions, ranging from physical barriers like skin to complex immune responses. These defenses are a testament to the ongoing evolutionary arms race between hosts and parasites.
  10. Plant-Parasite Interactions: Plants, too, are not immune to parasitic threats. They have developed a range of chemical and physical defenses to ward off parasitic plants and insects. These interactions highlight the universality of parasitism across the tree of life.

In conclusion, the realm of parasitism within evolutionary ecology offers profound insights into the adaptive strategies, co-evolutionary dynamics, and survival tactics employed by organisms. It underscores the intricate balance of competition and cooperation that shapes the natural world.

Examples of Parasitism

Parasitism, a symbiotic interaction, is characterized by one organism (the parasite) benefiting at the detriment of another organism (the host). This relationship can result in harm, and in some cases, the demise of the host. Herein, we delve into various instances of parasitism observed in humans, plants, insects, and fish.

1. Human Parasitism: Humans, as hosts, are susceptible to a myriad of parasitic invasions, some of which include:

  • Malaria: Caused by the Plasmodium species, this disease is vectored by mosquitoes. Infected individuals may exhibit fever, anemia, and potential organ impairment.
  • Tapeworms (Taenia species): Residing in the human gastrointestinal tract, these helminths siphon nutrients, potentially leading to malnutrition and other health complications.
  • Head Lice (Pediculus humanus capitis): These blood-sucking ectoparasites colonize the human scalp, inducing itching and discomfort.

2. Plant Parasitism: Certain plants have evolved to derive sustenance from other plants, including:

  • Dodder (Cuscuta species): A chlorophyll-deficient parasitic plant, dodder siphons nutrients from its host.
  • Broomrape (Orobanche species): This root parasite can stunt the growth and diminish the yield of its host plants.
  • Mistletoe (Viscum album): A semi-parasitic plant, mistletoe extracts water and nutrients from its arboreal hosts.
  • Hydnora africana: A subterranean parasite, it attaches to host roots for nourishment.

Plant parasites can further be categorized as:

  • Hemi Parasitic: Capable of photosynthesis but also extract nutrients from hosts.
  • Holo Parasitic: Entirely reliant on hosts due to their inability to photosynthesize.

3. Insect Parasitism: The insect realm is rife with parasitic interactions, such as:

  • Parasitic Wasps: These wasps oviposit within or on insects like caterpillars, with the emerging larvae consuming the host.
  • Fleas: Blood-feeding ectoparasites, fleas infest various mammals, including humans.
  • Lice: Residing on mammalian hair or avian feathers, lice are ectoparasites that feed on skin secretions.
  • Ticks: These arachnids attach to avian or mammalian skin, siphoning blood and potentially vectoring diseases.

4. Fish Parasitism: Fish are hosts to a plethora of parasites, with some notable examples being:

  • Copepods, Nematodes, and Leeches: These parasites colonize fish gills, deriving sustenance from the host.
  • Cymothoa exigua: An isopod that replaces the host fish’s tongue, feeding on the fish’s blood and mucus.
  • Cleaner Fish: While not parasites, cleaner fish, such as the bluestreak cleaner wrasses, remove dead skin and parasites from other fish.

Consumption of raw fish, especially in dishes like sushi, can pose a risk of parasitic infections in humans. However, in developed regions, such infections are uncommon, and precautionary measures, like freezing fish, are employed to mitigate risks.

In summation, parasitism is a multifaceted biological interaction observed across various taxa. Understanding these interactions is pivotal for both ecological studies and human health considerations.

Why Do We Need Parasites?

It’s tempting to believe that parasites should be removed since they hurt their hosts. At least fifty percent of all known species are parasitic. Parasites play an essential role within an ecosystem. They aid in controlling dominant species, hence promoting competition and diversity. Parasites provide an evolutionary purpose by transferring genetic material between species. In general, the presence of parasites is indicative of a healthy ecology.

Disadvantages of Parasitism

Parasitism, by its very nature, involves one organism benefiting at the expense of another. Here are some of the primary disadvantages associated with parasitism:

  1. Host Health Degradation:
    • Parasites can cause a range of health problems in their hosts, from mild discomfort to severe diseases. They can extract vital nutrients, leading to malnutrition, weakness, and susceptibility to other diseases.
  2. Mortality:
    • Some parasitic infections can be fatal to the host, especially if left untreated. This can lead to a decline in host populations, potentially affecting ecosystem dynamics.
  3. Economic Impact:
    • Parasites can have significant economic consequences, especially in the agricultural sector. Livestock and crops affected by parasites can lead to reduced yields, increased veterinary and medical costs, and loss of market value.
  4. Behavioral Changes:
    • Some parasites can alter their host’s behavior, making them more vulnerable to predators or leading them to environments that are more favorable for the parasite but potentially dangerous for the host.
  5. Reduced Reproductive Success:
    • Parasitic infections can reduce the reproductive success of hosts by causing sterility, reducing fertility, or impairing the host’s ability to care for its offspring.
  6. Resource Competition:
    • Parasites compete with hosts for essential resources like nutrients. This can lead to stunted growth, reduced energy levels, and overall diminished health of the host.
  7. Transmission of Secondary Infections:
    • Some parasites can act as vectors, transmitting other pathogens to their hosts. For example, mosquitoes can transmit diseases like malaria, dengue, and Zika virus.
  8. Ecosystem Imbalance:
    • If a parasite causes a significant decline in a host population, it can lead to imbalances in the ecosystem, affecting food chains and the overall health of the environment.
  9. Evolutionary Pressure:
    • The constant battle between hosts and parasites can lead to an evolutionary arms race, where both try to outcompete the other. While this can drive evolution, it can also lead to increased virulence in parasites and potential extinction events for certain host species.
  10. Social and Cultural Impact:
    • Parasitic diseases can lead to social stigma, isolation, and cultural challenges. Affected individuals might be ostracized or face discrimination, leading to psychological and societal issues.

In summary, parasitism poses numerous challenges to individual organisms, populations, and entire ecosystems. Understanding these disadvantages is crucial for effective disease management, conservation efforts, and ensuring the health and well-being of affected communities and ecosystems.

Importance of Parasitism

Parasitism, while often viewed negatively due to its detrimental effects on hosts, plays a crucial role in various ecological and evolutionary processes. Here are some of the significant aspects that highlight the importance of parasitism:

  1. Biodiversity and Ecosystem Dynamics:
    • Parasites contribute to biodiversity by adding to the complexity of food webs. They can affect predator-prey dynamics and influence the relative abundance of different species in an ecosystem.
  2. Population Control:
    • Parasites can regulate host populations, preventing them from reaching numbers that might lead to overexploitation of resources or habitat degradation. This can maintain a balance within ecosystems and ensure the survival of various species.
  3. Host Evolution and Adaptation:
    • The constant battle between hosts and parasites can drive evolutionary changes. Hosts evolve defense mechanisms, and in response, parasites develop counter-strategies. This “arms race” can lead to rapid evolutionary changes and speciation.
  4. Nutrient Cycling:
    • Parasites can influence nutrient cycling in ecosystems. For instance, when parasites cause disease or death in their hosts, they can affect the decomposition process and the return of nutrients to the soil.
  5. Behavioral Changes in Hosts:
    • Some parasites can alter the behavior of their hosts in ways that increase the parasite’s chances of survival and reproduction. This can have cascading effects on predator-prey interactions and overall ecosystem dynamics.
  6. Biological Indicators:
    • The presence or absence of certain parasites can serve as indicators of environmental health, water quality, or the presence of particular host species.
  7. Medical and Scientific Research:
    • Studying parasites has led to advances in medical research, particularly in understanding immune responses and developing treatments for parasitic infections.
  8. Hygiene Hypothesis:
    • Some researchers believe that certain parasitic infections might reduce the risk of autoimmune diseases in humans. The hygiene hypothesis suggests that exposure to parasites during childhood can modulate the immune system, reducing the risk of allergies and autoimmune disorders.
  9. Coevolution and Symbiotic Relationships:
    • Over time, some parasitic relationships can evolve into mutualistic or commensal relationships. This showcases the dynamic nature of interactions between organisms and how they can change over evolutionary timescales.
  10. Economic and Agricultural Implications:
    • Understanding parasitism is crucial for agriculture, as many parasites can affect crop yield and livestock health. By studying these interactions, we can develop strategies to manage and reduce the impact of parasitic infections.

In conclusion, while parasitism can have detrimental effects on individual hosts, it plays a vital role in shaping ecosystems, driving evolutionary processes, and maintaining the balance of nature. Recognizing its importance can help in conservation efforts, agricultural practices, and medical research.

Quiz Practice

What type of symbiotic relationship is parasitism?
a) Mutualism
b) Commensalism
c) Parasitism
d) Predation

Which of the following organisms can be a host in a parasitic relationship?
a) Plants
b) Animals
c) Both plants and animals
d) Neither plants nor animals

What is the primary characteristic of a parasite?
a) It benefits while the host is harmed.
b) It benefits while the host also benefits.
c) It neither harms nor benefits the host.
d) It is always microscopic in size.

Which of the following is NOT a type of parasitism?
a) Brood parasitism
b) Mutualism
c) Ectoparasitism
d) Endoparasitism

Which of the following parasites can infect the human circulatory system?
a) Tapeworm
b) Malaria parasite
c) Lice
d) Ticks

What can be a potential outcome of a severe parasitic infection in a host?
a) Enhanced immunity
b) Death
c) Increased reproduction
d) No noticeable effects

Which of the following is an example of an ectoparasite?
a) Tapeworm
b) Flea
c) Hookworm
d) Amoeba

Parasitism is believed to affect how many of the linkages in food webs?
a) One-quarter
b) One-half
c) Three-quarters
d) All of them

Which of the following diseases is caused by a parasitic protozoan?
a) Tuberculosis
b) Malaria
c) Influenza
d) Measles

In which type of environment are hosts more likely to encounter parasites?
a) Deserts
b) Polar regions
c) Tropical rainforests
d) Temperate grasslands


What is parasitism?

Parasitism is a type of symbiotic relationship where one organism, the parasite, benefits at the expense of another organism, the host.

How does parasitism differ from other symbiotic relationships?

In parasitism, only the parasite benefits while the host is harmed. In mutualism, both organisms benefit, and in commensalism, one organism benefits while the other is neither harmed nor benefited.

Can plants be hosts to parasites?

Yes, both plants and animals can be hosts to parasites. For instance, certain fungi and insects can parasitize plants.

What are the main types of parasitism?

The two main types are ectoparasitism, where the parasite lives on the external surface of the host, and endoparasitism, where the parasite lives inside the host’s body.

Are all parasites harmful?

While many parasites cause harm to their hosts, some have a neutral or even beneficial effect, especially when they have co-evolved with their hosts over a long period.

How do parasites reproduce?

Parasites can reproduce both sexually and asexually, depending on the species and its life stage. Some have complex life cycles involving multiple hosts.

Can parasitism lead to the death of the host?

In some cases, yes. If the parasitic infestation is severe, it can lead to the host’s death. However, it’s often in the parasite’s best interest to keep the host alive for as long as possible.

What are some common examples of human parasites?

Examples include tapeworms, lice, mites, and certain protozoans like the one causing malaria.

How can parasitic infections be prevented or treated?

Preventative measures include good hygiene practices, avoiding contact with contaminated water or food, and using protective measures in areas with high parasite prevalence. Treatments vary depending on the parasite and include various medications and sometimes surgical intervention.

Are there any benefits to having parasites?

Some studies suggest that certain parasitic infections might reduce the risk of autoimmune diseases. This is known as the hygiene hypothesis, which proposes that a lack of exposure to infectious agents in early childhood can increase susceptibility to allergic diseases.


  • Rohde, K. (2013). Parasitism. Encyclopedia of Biodiversity, 656–673. doi:10.1016/b978-0-12-384719-5.00103-9 
  • Olano JP, Weller PF, Guerrant RL, Walker DH. Principles of Parasitism: Host–Parasite Interactions. Tropical Infectious Diseases: Principles, Pathogens and Practice. 2011:1–7. doi: 10.1016/B978-0-7020-3935-5.00001-X. Epub 2011 Apr 29. PMCID: PMC7149714.

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