In biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life.[1] The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one".[2] Parasites include protozoa such as the agents of malaria, sleeping sickness, and amoebic dysentery; animals such as hookworms, lice, and mosquitoes; plants such as mistletoe and dodder; and fungi such as honey fungus and ringworm. There are six major evolutionary strategies within parasitism, namely parasitic castrator, directly transmitted parasite, trophically transmitted parasite, vector-transmitted parasite, parasitoid, and micropredator.

Unlike predators, parasites, with the exception of parasitoids, typically do not kill their host, are generally much smaller than their host, and often live in or on their host for an extended period. Parasitism is a type of consumer-resource interaction.[3] Parasites of animals show a high degree of specialization, and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the Plasmodium species, and fleas.

Parasites reduce host biological fitness by general or specialized pathology, from parasitic castration and impairment of secondary sex characteristics to modification of host behavior. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular transmission. Although parasitism is often unambiguous, it is part of a spectrum of interactions between species, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic.

People have known about parasites such as roundworms and tapeworms since ancient Egypt, Greece, and Rome. In Early Modern times, Antonie van Leeuwenhoek observed Giardia lamblia in his microscope in 1681, while Francesco Redi described endo- and ectoparasites including sheep liver fluke and ticks. Modern parasitology developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to satirical effect in Jonathan Swift's 1733 poem "On Poetry: A Rhapsody", comparing poets to hyperparasitical "vermin". In fiction, Bram Stoker's 1897 Gothic horror novel Dracula and its many later adaptations featured a blood-drinking parasite. Ridley Scott's 1979 film Alien was one of many works of science fiction to feature a terrifying[4] parasitic alien species.


First used in English in 1539, the word parasite comes from the Medieval French parasite, from the Latin parasitus, the latinisation of the Greek παράσιτος (parasitos), "one who eats at the table of another"[5] and that from παρά (para), "beside, by"[6] + σῖτος (sitos), "wheat", hence "food".[7] The related term parasitism appears in English from 1611.[8]

Evolutionary strategies

Parasitism is one of several kinds of symbiosis, close and persistent biological interactions. It is distinguished from commensalism and mutualism by the harm done to the host by the parasite.[9] Predation is not generally considered a symbiosis as the interaction is brief, but the entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one".[2] Within that scope are many possible ways of life. Parasites are classified in a variety of different but overlapping schemes, based on their interactions with their hosts and on their life cycles. An obligate parasite is totally dependent on the host to complete its life cycle, while a facultative parasite is not. A direct parasite has only one host while an indirect parasite has multiple hosts. For indirect parasites, there will always be a definitive host and an intermediate host.[10][11]

Basic strategies

The parasitic castrator Sacculina carcini (highlighted) attached to a crab

There are six basic evolutionary strategies within parasitism, namely parasitic castrator, directly transmitted parasite, trophically transmitted parasite, vector-transmitted parasite, parasitoid, and micropredator. These apply to parasites whose hosts are plants as well as animals:[12][13] These strategies are adaptive peaks; many intermediate strategies such as mesoparasitism (between endo- and ectoparasitism) are possible,[14] but organisms in many different groups have consistently converged on these six, which are evolutionarily stable.[12]

Parasitic castrators

Human head lice are obligate directly-transmitted ectoparasites.

Parasitic castrators destroy their hosts' ability to reproduce, diverting the energy that would have gone into reproduction into host growth, with gigantism a common outcome. The hosts' other systems are left intact, allowing it to survive and sustain the parasite.[12] Parasitic crustaceans such as Sacculina specifically cause damage to the gonads of their host crabs. In the case of Sacculina, the testes of over two thirds of their crab hosts degenerate sufficiently for these male crabs to have gained female secondary sex characteristics such as broader abdomens, smaller claws (chelae) and egg-grasping appendages.[15] The trematode Zoogonus lasius causes parasitic castration of the intertidal snail Ilyanassa obsoleta; other trematodes directly or indirectly castrate other species of snail.[15]

Directly transmitted ectoparasites

Directly transmitted ectoparasites, living on the outside of their hosts, rely on chance encounters with members of their host species to feed and reproduce. They may spread from one host to another through skin-to-skin contact, or lie dormant until a host steps on or brushes against them.[12] Examples include lice, fleas, and ticks.[16] Some such as bird parasites frequent nests, to which their hosts are likely to return.[17][18][19]

Schistosoma mansoni is an obligate endoparasite, causing schistosomiasis (bilharzia).

Trophically transmitted endoparasites

Trophically transmitted endoparasites such as many parasitic worms (helminths) have a life cycle involving two or more hosts. In their juvenile stages, they infect and often encyst in the intermediate host. When this animal is eaten by a predator, the definitive host, the parasite survives the digestion process and matures into an adult; some live as intestinal parasites, others in other intercellular spaces within the body. Some parasites modify the behavior of their intermediate hosts, increasing their chances of being eaten by a predator.[12] Coinfection by multiple parasites is common.[20] With autoinfection, the infection of a primary host with a helminth such as Strongyloides stercoralis, the whole of the parasite's life cycle takes place in a single organism.[21]

Vector-transmitted endoparasites

The vector-transmitted protozoan parasite Trypanosoma among human red blood cells

Vector-transmitted endoparasites rely on a third party to carry them from one host to another. These are often microscopic non-animal parasites, namely protozoa, bacteria, or viruses, often living inside the cells of their hosts as disease-causing pathogens. Their vectors are mostly parasitic arthropods such as fleas, lice, ticks and mosquitoes.[12][22]


Parasitoids are insects which sooner or later kill their hosts, so this form of parasitism is close to predation. The great majority of parasitoids are hymenopterans, parasitoid wasps. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts.[23]

Idiobiont parasitoids are usually ectoparasites, stinging their often large prey on capture, either killing them outright or paralyzing them immediately. The immobilised prey is then carried to a nest, sometimes alongside other prey if they are not individually large enough to support a parasitoid throughout its development. An egg is laid on top of the prey, and the nest is then sealed. The parasitoid develops rapidly through its larval and pupal stages, feeding on the provisions left for it.[23]

Koinobiont parasitoids are usually endoparasites, laying their eggs inside young hosts, usually larvae. These are allowed to go on growing, so the host and parasitoid develop together for an extended period. Some koinobionts regulate their host's development with hormones, for example preventing it from pupating or making it moult whenever the parasitoid is ready to moult.[23]


Mosquitoes are micropredators, and important vectors of disease.

Micropredators actively hunt for hosts, like traditional predators. However, they choose hosts that are large but unable to resist attack. For example, mosquitoes attack animals too slow to protect themselves from their bite, and serve as vectors of diseases caused by protozoan and other parasites.[12] Similarly, phytophagous scale insects, aphids, and caterpillars attack much larger plants, and serve as vectors of bacteria, fungi and viruses causing plant diseases, and plants defoliated by caterpillars may die, as in parasitoidism. Female scale insects are unable to move, so they are obligate parasites, permanently attached to their hosts.[13]


Among the many variations on parasitic strategies are hyperparasitism,[24] social parasitism,[25] brood parasitism,[26] kleptoparasitism,[27] and sexual parasitism.[28]


Hyperparasites feed on another parasite, as exemplified by protozoa living in helminth parasites,[24] or facultative or obligate parasitoids whose hosts are either parasites or parasitoids.[12][23] Another variant is adelpho-parasitism, where the host species is closely related to the parasite, often in same family or genus, as in the citrus blackfly parasitoid, Encarsia perplexa, unmated females of which may lay haploid eggs in the fully developed larvae of their own species, producing male offspring,[29] while the marine worm Bonellia viridis has a similar reproductive strategy, although the larvae are planktonic.[30]

Parasitism can take the form of isolated cheating or exploitation among more generalized mutualistic interactions. For example, broad classes of plants and fungi exchange carbon and nutrients in common mutualistic mycorrhizal relationships; however, some myco-heterotrophic plants cheat by taking carbon from a fungus rather than donating it.[31]

Social parasitism

Social parasites take advantage of interactions between members of social organisms such as ants, termites, and bumblebees. Examples include the large blue butterfly, Phengaris arion, its larvae employing ant mimicry (myrmecomorphy) to parasitize certain ants,[25] Bombus bohemicus, a bumblebee which invades the hives of other bees and takes over reproduction while their young are raised by host workers, and Melipona scutellaris, a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen.[32] An extreme example of social parasitism is found in the ant Tetramorium inquilinum, an obligate parasite which lives exclusively on the backs of other Tetramorium ants.[33] Emery's rule notes that social parasites tend to be closely related to their hosts, often being in the same genus.[34][35]

Intraspecific social parasitism occurs in parasitic nursing, where some individual young take milk from unrelated females. In wedge-capped capuchins, higher ranking females sometimes take milk from low ranking females without any reciprocation.[36]

Brood parasitism

In brood parasitism, the hosts behave as unwitting babysitters as they raise the young as their own. Brood parasites include birds in different families such as cowbirds, whydahs, cuckoos, and black-headed ducks. These do not build nests of their own, but leave their eggs in nests of other species. The eggs of some brood parasites mimic those of their hosts, implying selection by the hosts against parasitic eggs.[26][37] The adult female European cuckoo further mimics a predator, the European sparrowhawk, giving her time to lay her eggs in the host's nest unobserved.[38]


In kleptoparasitism (from Greek κλέπτης (kleptēs), "thief"), parasites steal food gathered by the host. The parasitism is often on close relatives, whether within the same species or between species in the same genus or family. For instance, the many lineages of cuckoo bees lay their eggs in the nest cells of other bees in the same family.[27] Kleptoparasitism is uncommon but conspicuous in birds; some such as skuas are specialised in pirating food from other seabirds, relentlessly chasing them down until they disgorge their catch.[39]

Sexual parasitism

In many animals, males are much smaller than females. In some species of anglerfish, such as Ceratias holboelli, the males are so small they have become sexual parasites, wholly dependent on females of their own species for survival, and unable to fend for themselves. The female nourishes the male and protects him from predators, while the male gives nothing back except the sperm that the female needs to produce the next generation.[28]

Taxonomic range

Head (scolex) of tapeworm Taenia solium, an intestinal parasite, has hooks and suckers to attach to its host.

Parasitism occurs in a wide range of organisms, including animals,[40] plants,[41] fungi,[42] protozoa,[43] bacteria,[44] and viruses.[45]


Parasitism is widespread in the animal kingdom,[46] and has evolved independently from free-living forms hundreds of times.[12] Many species of helminth including trematodes and cestodes have complex life-cycles involving two or more hosts. By far the largest group is the parasitoid wasps in the Hymenoptera.[12] The phyla and classes with the largest numbers of parasitic species are listed in the table. Numbers are conservative minimum estimates. The columns for Endo- and Ecto-parasitism refer to the definitive host, as documented in the Vertebrate and Invertebrate columns.[40]

Cuscuta (a dodder), a stem holoparasite, on an acacia tree


A parasitic plant derives some (hemiparasites such as mistletoe) or all of its nutritional requirements (holoparasites such as dodder) from another living plant. They make up about 1% of angiosperms and are in almost every biome in the world.[41] All parasitic plants have modified roots, haustoria, which penetrate the host plants, connecting them to the conductive system – either the xylem, the phloem, or both. This provides them with the ability to extract water and nutrients from the host. Parasitic plants are classified depending on where the parasitic plant latches onto the host – stem or root – and the amount of nutrients it requires. Since holoparasites have no chlorophyll and therefore cannot make food for themselves by photosynthesis, they are always obligate parasites, deriving all their food from their hosts.[41] Some parasitic plants can locate their host plants by detecting chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plant in approximately 20 families of flowering plants are known.[48][41]

Species within Orobanchaceae (broomrapes) are some of the most economically destructive species on Earth. Species of Striga (witchweeds) are estimated to cost billions of dollars a year in crop yield loss annually, infesting over 50 million hectares of cultivated land within Sub-Saharan Africa alone. Striga infects both grasses and grains, including corn, rice and sorghum, undoubtedly some of the most important food crops. Orobanche also threatens a wide range of important crops, including peas, chickpeas, tomatoes, carrots, and varieties of the genus Brassica (cabbages). Yield loss from Orobanche can reach 100%; despite extensive research, no method of control has been entirely successful.[49]

The honey fungus, Armillaria mellea, is a parasite of trees, and a saprophyte feeding on the trees it has killed.


Parasitic fungi derive some or all of their nutritional requirements from plants, other fungi, or animals, and unlike mycorrhizal fungi which have a mutualistic relationship with their host plants, they are pathogenic. For example, the honey fungi in the genus Armillaria grow in the roots of a wide variety of trees, and eventually kill them. They then continue to live in the dead wood, feeding saprophytically.[42]

Borrelia burgdorferi, the bacterium that causes Lyme disease, is transmitted by Ixodes ticks.


Protozoa such as Plasmodium, Trypanosoma, and Giardia[50] are endoparasitic. They cause serious diseases in vertebrates including humans – in these examples, malaria, sleeping sickness, and a form of amoebic dysentery respectively – and have complex life-cycles.[43]


Many bacteria are parasitic, though they are generally thought of as pathogens (causes of disease) instead.[44] Parasitic bacteria are extremely diverse, and infect their hosts by a variety of routes. To give a few examples, Bacillus anthracis, the cause of anthrax, is spread by contact with infected domestic animals; the bacillus's spores, which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. Borrelia, the cause of Lyme disease and relapsing fever, is transmitted by a vector, ticks of the genus Ixodes, from the diseases' reservoirs in animals such as deer. Campylobacter jejuni, a cause of severe enteritis (gut inflammation), is spread by the fecal-oral route from animals, or by eating insufficiently cooked poultry, or by contaminated water. Haemophilus influenzae, an agent of bacterial meningitis and respiratory tract infections such as influenza and bronchitis, is transmitted by droplet contact. Treponema pallidum, the cause of syphilis, is spread by sexual intercourse.[51]

Enterobacteria phage T4 is a bacteriophage virus. It infects its host, Escherichia coli, by injecting its DNA through its tail, which attaches to the bacterium's surface.


Viruses are obligate intracellular parasites, characterized by extremely limited biological function, to the point where, while they are evidently able to infect all other organisms from bacteria and archaea to animals, plants and fungi, it is unclear whether they can themselves be described as living. Viruses consist of a strip of genetic material (DNA or RNA), covered in a protein coat and sometimes a lipid envelope. They thus lack all the usual machinery of the cell such as enzymes, relying entirely on the host cell's ability to replicate DNA and synthesise proteins. Most viruses are bacteriophages, infecting bacteria, and it is possible that viruses are both extremely ancient, being at least as old as the first cells, and polyphyletic, different groups of viruses having evolved from several entirely unrelated ancestors.[45][52][53][54]


Life cycle of Entamoeba histolytica, an anaerobic parasitic protozoan transmitted by the fecal-oral route

Parasites use a variety of methods to infect their hosts, including physical contact, the fecal-oral route, free-living infectious stages, and insect vectors, suiting their differing hosts and ecological contexts.[55] Examples to illustrate some of the possible combinations are given in the table.

Examples of transmission methods in different ecological contexts[55]
Parasite Host Transmission method Ecological context
Gyrodactylus turnbulli
(a trematode)
Poecilia reticulata
physical contact social behavior
e.g. Strongyloides
Macaca fuscata
(Japanese macaque)

social behavior (grooming)

(a nematode)
Apodemus flavicollis
(yellow-necked mouse)
fecal-oral sex-biased transmission (mainly to males)
(a tick)
Sphenodon punctatus
free-living infectious stages social behavior
(malaria parasite)
Birds, mammals
(inc. humans)
Anopheles mosquito vector, attracted by odour of infected human host[56]

Among protozoan endoparasites, such as the malarial parasites in the genus Plasmodium and sleeping sickness parasites in the genus Trypanosoma, infective stages in the host's blood are transported to new hosts by biting blood-drinking (hematophagous) insects acting as vectors.[43]

Host defences

Hosts have evolved a variety of defensive measures against their parasites, including physical barriers like the skin of vertebrates,[57] the immune system of mammals,[58] insects actively removing parasites,[59] and defensive chemicals in plants.[60]

Biologists such as W. D. Hamilton have suggested that genetic recombination through sexual reproduction could have evolved to help to defeat multiple parasites, showing with mathematical modelling that sexual reproduction would be evolutionarily stable even under unpromising conditions, and that the theory's predictions match the actual ecology of sexual reproduction.[61][62] However, there may be a trade-off between immune defence and secondary sex characteristics in breeding male vertebrate hosts, such as the plumage of peacocks and the manes of male lions. This is because the male hormone testosterone encourages the growth of secondary sex characteristics, favouring such males in sexual selection, at the price of reducing their immune defences.[63]


The dry skin of vertebrates such as the short-horned lizard prevents entry of many parasites.

The physical barrier of the tough and often dry and waterproof skin of reptiles, birds and mammals keeps invading microorganisms from entering the body. Human skin also secretes sebum, which is toxic to most microorganisms.[57] On the other hand, larger parasites such as trematodes detect chemicals produced by the skin to locate their hosts when they enter the water. Vertebrate saliva and tears contain lysozyme, an enzyme which breaks down cell walls of invading bacteria.[57] Should the organism pass the mouth, the stomach with its hydrochloric acid, toxic to most microorganisms, is the next line of defence.[57] Some intestinal parasites have a thick, tough outer coating which is digested slowly or not at all, allowing the parasite to pass through the stomach alive, at which point they enter the intestine and begin the next stage of their life. Once inside the body, parasites must overcome the immune system's serum proteins and pattern recognition receptors, intracellular and cellular, that trigger the adaptive immune system's lymphocytes such as T cells and antibody-producing B cells. These have receptors that recognize parasites.[58]


Leaf spot on oak. The spread of the parasitic fungus is limited by defensive chemicals produced by the tree, resulting in circular patches of damaged tissue.

Insects often adapt their nests to reduce parasitism. For example, one of the key reasons why the wasp Polistes canadensis nests across multiple combs, rather than building a single comb like much of the rest of its genus, is to avoid infestation by tineid moths. The tineid moth lays its eggs within the wasps' nests and then these eggs hatch into larvae that can burrow from cell to cell and prey on wasp pupae. Adult wasps attempt to remove and kill moth eggs and larvae by chewing down the edges of cells, coating the cells with an oral secretion that gives the nest a dark brownish appearance.[59]


Plants respond to parasite attack with a series of chemical defences, such as the jasmonic acid-insensitive (JA) and NahG (SA) pathways.[60] Different biochemical pathways are activated by different parasites.[64] In general, plants can either initiate a specific or a non-specific response.[65]

Specific responses involve recognition of a parasite by the plant's cellular receptors, leading to a strong but localized response: defensive chemicals are produced around the area where the parasite was detected, blocking its spread, and avoiding wasting defensive production where it is not needed.[65]

Nonspecific defensive responses are systemic, meaning that the responses are not confined to an area of the plant, but spread throughout the plant, making them costly in energy. These are effective against a wide range of parasites.[65]

Evolutionary ecology

Restoration of a Tyrannosaurus with holes possibly caused by a Trichomonas-like parasite

Parasitism has arisen independently many times. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles,[66] and it is frequent in plants and fungi. Almost all free-living animals are host to one or more parasitic taxa.[66] Humans, for example, have 342 species of helminth parasite, and 70 species of protozoan parasite.[67] This is harder to demonstrate from the fossil record, but for example holes in the skulls of several specimens of Tyrannosaurus may have been caused by Trichomonas-like parasites.[68]


Coevolution favoring mutualism

The gram-negative bacterium Wolbachia within an insect cell

Long-term coevolution sometimes leads to a relatively stable relationship tending to commensalism or mutualism, as, all else being equal, it is in the evolutionary interest of the parasite that its host thrives. A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite—to the point that the parasite's absence causes the host harm. For example, although animals infected with parasitic worms are often clearly harmed, and therefore parasitized, such infections may also reduce the prevalence and effects of autoimmune disorders in animal hosts, including humans.[69] In a more extreme example, some nematode worms cannot reproduce, or even survive, without infection by Wolbachia bacteria.[70]

Lynn Margulis and others have argued, following Peter Kropotkin's 1902 Mutual Aid: A Factor of Evolution, that natural selection drives relationships from parasitism to mutualism when resources are limited. This process may have been involved in the symbiogenesis which formed the eukaryotes from an intracellular relationship between archaea and bacteria, though the sequence of events remains largely undefined.[71][72]

Competition favoring virulence

Competition between parasites can be expected to favor faster reproducing and therefore more virulent parasites, by natural selection.[73][74] Parasites whose life cycle involves the death of the host, to exit the present host and sometimes to enter the next, evolve to be more virulent, and may alter the behavior or other properties of the host to make it more vulnerable to predators.[75] Conversely, parasites whose reproduction is largely tied to their host's reproductive success tend to become less virulent or mutualist, so that their hosts reproduce more effectively.[75]

The protozoan Toxoplasma gondii facilitates its transmission by inducing behavioral changes in rats through infection of neurons in their central nervous system.

Among competing parasitic insect-killing bacteria of the genera Photorhabdus and Xenorhabdus, virulence depended on the relative potency of the antimicrobial toxins (bacteriocins) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when caterpillars were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.[73]


Biologists long suspected cospeciation of flamingos and ducks with their parasitic lice, which were similar in the two families. Cospeciation did occur, but it led to flamingos and grebes, with a later host switch of flamingo lice to ducks.

A parasite sometimes undergoes co-speciation with its host, resulting in the pattern described in Fahrenholz's rule, that the phylogenies of the host and parasite come to mirror each other.[76]

An example is between the simian foamy virus (SFV) and its primate hosts. The phylogenies of SFV polymerase and the mitochondrial cytochrome oxidase subunit II from African and Asian primates were found to be closely congruent in branching order and divergence times, implying that the simian foamy viruses co-speciated with Old World primates for at least 30 million years.[77]

The presumption of a shared evolutionary history between parasites and hosts can help elucidate how host taxa are related. For instance, there has been a dispute about whether flamingos are more closely related to storks or ducks. The fact that flamingos share parasites with ducks and geese was initially taken as evidence that these groups were more closely related to each other than either is to storks. However, evolutionary events such as the duplication or extinction of parasite species (without similar events on the host phylogeny) often erode similarities between host and parasite phylogenies. In the case of flamingos, they have similar lice to those of grebes. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice then switched hosts to ducks, creating the situation which had confused biologists.[78]

Parasites infect hosts within their same geographical area (sympatric) more effectively, as has been shown with digenetic trematodes infecting lake snails.[79] This is in line with the Red Queen hypothesis, which states that interactions between species lead to constant natural selection for coadaptation. Parasites track the locally common hosts' phenotypes, so the parasites are less infective to allopatric hosts, those from different geographical regions.[79]

Modifying host behaviour

Some parasites modify host behaviour in order to increase the transmission between hosts, often in relation to predator and prey (parasite increased trophic transmission). For example, in California salt marshes, the fluke Euhaplorchis californiensis reduces the ability of its killifish host to avoid predators.[80] This parasite matures in egrets, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan Toxoplasma gondii, a parasite that matures in cats but can be carried by many other mammals. Uninfected rats avoid cat odors, but rats infected with T. gondii are drawn to this scent, which may increase transmission to feline hosts.[81] The malaria parasite modifies the skin odour of its human hosts, increasing their attractiveness to mosquitoes and hence improving the chance that the parasite will be transmitted.[56]

Bed bug, Cimex lectularius, is flightless, like many insect ectoparasites.

Trait loss

Parasites are able to exploit their hosts for a variety of functions, and so do not need to carry out those activities themselves. Parasites which lose those functions then have a selective advantage, as they can divert resources to reproduction. Many insect ectoparasites including bedbugs, batbugs, lice and fleas have lost their ability to fly, relying instead on their hosts for transport.[82] Trait loss more generally is widespread among parasites.[83]

Biology and conservation

Ecology and parasitology

Parasitism and parasite evolution were until the twentyfirst century studied by parasitologists, in a science dominated by medicine, rather than by ecologists or evolutionary biologists. Even though parasite-host interactions were plainly ecological, the history of parasitology caused what the evolutionary ecologist Robert Poulin called a "takeover of parasitism by parasitologists", leading ecologists to ignore the area. This was in his opinion "unfortunate", as parasites are "omnipresent agents of natural selection" and significant forces in evolution and ecology. The long-standing split between the sciences limited the exchange of ideas, with separate conferences and separate journals. The technical languages of ecology and parasitology sometimes involved different meanings for the same words. There were philosophical differences, too: Poulin notes that, influenced by medicine, "many parasitologists accepted that evolution led to a decrease in parasite virulence, whereas modern evolutionary theory would have predicted a greater range of outcomes".[84]

Their complex relationships make parasites difficult to place in food webs: a trematode with multiple hosts for its various life-cycle stages would occupy many positions in a food web simultaneously, and would set up loops of energy flow, confusing the analysis. Further, since nearly every animal has (multiple) parasites, parasites would occupy the top levels of every food web.[67]

The rescuing from extinction of the California condor was a successful if very expensive project, but its ectoparasite, the louse Colpocephalum californici, became extinct.

Rationale for conservation

Although parasites are widely considered to be harmful, the eradication of all parasites would not be beneficial. Parasites account for at least half of life's diversity; they perform important ecological roles; and without parasites, organisms might tend to asexual reproduction, diminishing the diversity of traits brought about by sexual reproduction.[85] Parasites provide an opportunity for the transfer of genetic material between species, facilitating evolutionary change.[75] Many parasites require multiple hosts of the different species to complete their life cycles and rely on predator-prey or other stable ecological interactions to get from one host to another. The presence of parasites thus indicates that an ecosystem is healthy.[86]

A well-known case was that of an ectoparasite, the California condor louse, Colpocephalum californici. Any lice found were "deliberately killed" during the major and very costly captive breeding program to rescue its host, the Californian condor. The result was that the condor was saved, and returned to the wild, while the parasite became extinct.[87]

Although parasites are often omitted in depictions of food webs, they usually occupy the top position. Parasites can function like keystone species, reducing the dominance of superior competitors and allowing competing species to co-exist.[67][88][89]

Quantitative ecology

A single parasite species usually has an aggregated distribution across host individuals, which means that most hosts harbor few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology, as it renders parametric statistics as commonly used by biologists invalid. Log-transformation of data before the application of parametric test, or the use of non-parametric statistics is recommended by several authors, but this can give rise to further problems, so quantitative parasitology is based on more advanced biostatistical methods.[90]


Cyst and imago of Giardia lamblia, the protozoan parasite that causes giardiasis, first observed by Antonie van Leeuwenhoek in 1681


Human parasites including roundworms, the Guinea worm, threadworms and tapeworms are mentioned in Egyptian papyrus records from 3000 BC onwards; the Ebers papyrus describes hookworm. In ancient Greece, parasites including the bladder worm are described in the Hippocratic Corpus, while the comic playwright Aristophanes called tapeworms "hailstones". The Roman physicians Celsus and Galen documented the roundworms Ascaris lumbricoides and Enterobius vermicularis.[91]


The Persian physician Avicenna recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms.[91]

Early Modern

A plate from Francesco Redi's Osservazioni intorno agli animali viventi che si trovano negli animali viventi (Observations on living animals found inside living animals), 1684

Antonie van Leeuwenhoek observed and illustrated Giardia lamblia in 1681, and linked it to "his own loose stools". This was the first protozoan parasite of humans that he recorded, and the first to be seen under a microscope.[91]

Francesco Redi described ecto- and endoparasites in his 1687 book Esperienze Intorno alla Generazione degl'Insetti (Experiences of the Generation of Insects), illustrating ticks, the larvae of nasal flies of deer, and sheep liver fluke. His 1684 book Osservazioni intorno agli animali viventi che si trovano negli animali viventi (Observations on Living Animals found in Living Animals) described and illustrated over 100 parasites including the human roundworm.[92] He noted that parasites develop from eggs, contradicting the theory of spontaneous generation.[93]

Birth of modern parasitology

Modern parasitology developed in the 19th century with accurate observations by several researchers and clinicians. In 1828, James Annersley described amoebiasis, protozoal infections of the intestines and the liver, though the pathogen, Entamoeba histolytica, was not discovered until 1873 by Friedrich Lösch. James Paget discovered the intestinal nematode Trichinella spiralis in humans in 1835. James McConnell described the human liver fluke in 1875. Patrick Manson discovered the life cycle of elephantiasis, caused by nematode worms transmitted by mosquitoes, in 1877. Manson further predicted that the malaria parasite, Plasmodium, had a mosquito vector, and persuaded Ronald Ross to investigate. Ross confirmed that the prediction was correct in 1897–1898. At the same time, Giovanni Battista Grassi and others described the malaria parasite's life cycle stages in Anopheles mosquitoes. Ross was controversially awarded the 1902 Nobel prize for his work, while Grassi was not.[91] In 1903, David Bruce identified the protozoan parasite and the tsetse fly vector of African trypanosomiasis.[94]


Given the importance of malaria, with some 220 million people infected annually, many attempts have been made to interrupt its transmission, such as by killing parasites in the blood with antimalarial drugs, by eradicating its mosquito vectors with organochlorine and other insecticides, or by developing a malaria vaccine. All of these have proven problematic, with drug resistance, insecticide resistance among mosquitoes, and repeated failure of vaccines as the parasite mutates.[95] The first and as of 2015 the only licenced vaccine for any parasitic disease of humans is RTS,S for Plasmodium falciparum malaria.[96]

Prophylactic usage and resistance

Poulin observes that the widespread prophylactic use of anthelmintic drugs in domestic sheep and cattle constitutes a worldwide uncontrolled experiment in the life-history evolution of their parasites. The outcomes depend on whether the drugs decrease the chance that a parasite larva (such as a nematode) will reach adulthood. If so, natural selection can be expected to favour the production of eggs at an earlier age. If on the other hand the drug mainly affects adult parasites, selection could cause delayed maturity and increased virulence. Such changes appear to be under way: the nematode Teladorsagia circumcincta is changing its adult size and fecundity in response to drugs.[97]

Cultural significance

"An Old Parasite in a New Form": an 1881 Punch cartoon by Edward Linley Sambourne compares a crinoletta bustle to a parasitic insect's exoskeleton

Classical times

In the classical era, the concept of the parasite was not strictly pejorative:[98] the parasitus was an accepted role in Roman society, in which a person could live off the hospitality of others, and in return provide "flattery, simple services, and a willingness to endure humiliation".[99][100]


Parasitism has a derogatory sense in popular usage. According to the immunologist John Playfair,[101]

In everyday speech, the term 'parasite' is loaded with derogatory meaning. A parasite is a sponger, a lazy profiteer, a drain on society.[101]

The satirical cleric Jonathan Swift refers to hyperparasitism in his 1733 poem "On Poetry: A Rhapsody", comparing poets to "vermin" who "teaze and pinch their foes":[102]

The vermin only teaze and pinch
Their foes superior by an inch.
So nat'ralists observe, a flea
Hath smaller fleas that on him prey;
And these have smaller fleas to bite 'em.
And so proceeds ad infinitum.
Thus every poet, in his kind,
Is bit by him that comes behind:


In Bram Stoker's 1897 Gothic horror novel Dracula, and its many film adaptations, the eponymous Count Dracula is a blood-drinking parasite. The critic Laura Otis argues that as a "thief, seducer, creator, and mimic, Dracula is the ultimate parasite. The whole point of vampirism is sucking other people's blood—living at other people's expense."[103]

Disgusting and terrifying parasitic alien species are widespread in science fiction,[104][105] as for instance in Ridley Scott's 1979 film Alien.[106][107] In one scene of that film, a Xenomorph bursts out of the chest of a dead man, with blood squirting out under high pressure assisted by explosive squibs. Animal viscera were used to reinforce the shock effect. The scene was filmed in a single take, and the startled reaction of the actors was genuine.[4][108]


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  108. ^ Nordine, Michael (25 April 2017). "'Alien' Evolution: Explore Every Stage in the Xenomorph's Gruesome Life Cycle. Celebrate Alien Day with a look at the past, present and future of cinema's most terrifying extraterrestrial". IndieWire. Nothing speaks to the xenomorph's visceral terror quite like the fact that this stage of its life cycle — which, true to its name, finds the creature literally bursting through its host's ribcage — isn't even its final form. For every alien that is born, another being (usually a human) is violently killed. And there's a reason the other actors look utterly terrified by what's happening in that infamous scene: Scott intentionally withheld key details from them in order to elicit genuine reactions. 

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