Parasitism is a form of symbiosis (living together). Unlike mutualism where both partners in the relationship mutually beneficial, in parasitism the host normally gains little or no benefit. Usually the host suffers harmful effects, although they may be minor. It is not normally in the parasite’s interests to kill its host, and ‘the good parasite does not harm its host’. Parasites can be broadly divided into ectoparasites and endoparasites.

Almost all animal and protoctistan phyla have parasitic members, and almost every individual animal is itself parasitized. Many parasites are themselves parasitized, a phenomenon called hyperparasitism: e.g. some platyhelminth parasites are, in turn, hosts to the protoctistan parasite Nosema sp. As the eighteenth-century satirist and author of Gulliver’s Travels, Jonathan Swift quipped, ‘So, naturalists observe, a flea hath smaller fleas that on him prey; and these have smaller fleas to bite ’em, and so proceed ad infinitum’.

Ectoparasites live on the outer surface of the host. Examples include the annelid leeches parasitizing fishes or mammals, the cirripede crustacean Sacculina sp. parasitizing the shore-crab Carcinus sp., fleas and lice (on birds and mammals, for instance) among the Insecta, the gastropod mollusc Brachystomia sp. on bivalve molluscs hosts, and the agnathan craniate lamprey on bony or cartilaginous fish hosts.

Endoparasites live within the gut, tissues or cells of the host. They include many Protoctista (e.g. the malaria parasite Plasmodium spp.) and members of several ‘worm’ phyla including the Platyhelminthes (flukes and tapeworms), Nematoda (roundworms) and Acanthocephala (spiny-headed worms). A mollusc endoparasite is the vermiform Entoconcha sp. that parasitizes echinoderm hosts. In some species of deep-sea angler fishes the male becomes a total endoparasite on the female, its body degenerating so the sperm-producing testis is virtually the only part of the male fish remaining.

Parasitoids are larvae of insects that always kill their hosts by ingesting their tissues – they can be endo- or ectoparasitoids, and include many wasp and fly spp. Some parasites are generalists and will parasitize several host species: these are known as polyxenous. Monoxenous parasite species (the majority) are specialists and only parasitize one host species.

Life cycles

Many parasites have complex life cycles associated with multiple larval stages, elaborate, sophisticated anatomical and physiological adaptations, and more than one host species. Parasite life cycles can be divided into direct life cycles and indirect life cycles: in the former there is only one host species in the cycle, in the latter more than one host species.

In direct life cycles, the parasite transfers from one definitive host to another definitive host without any intermediate host. An example is the monogenean fluke Polystoma intergerrimum (phylum Platyhelminthes) that lives in the bladder of frogs and toads. Transfer from host to host is through ingestion of eggs deposited in the urine or feces in the freshwater habitat. The nematode hookworm Acyclostoma duodenale that parasitizes humans and leads to severe anemia has a direct life cycle that includes a free-living stage in the soil and four larval stages. The transfer is through faeces and then by contaminated soil infecting a wound on the foot.

The digenean blood fluke Schistosoma haematobium is an example of a platyhelminth that has an indirect life cycle with an intermediate host of another species. Male and female adult flukes live in the human urinary bladder. The flukes mate and some of their eggs, which have sharp spines, remain in the human definitive host and eventually reach the lungs where they cause the debilitating and miserable disease schistosomiasis, formerly known as bilharziasis (it is estimated that almost 80% of the human population of parts of tropical Africa and Asia have this disease). Other eggs are voided in urine. The eggs hatch in freshwater. Aquatic snails (the intermediate host, sometimes called the primary [first] host) ingest the larvae (called miracidia). The larvae seem to be protected against ingestion in the gut of the correct intermediate host, although they are not protected in the guts of other, even closely related, snail species. The larva bores through the gut wall and forms a sporocyst.

Apomictic parthenogenesis (a form of asexual reproduction) within the sporocyst results in numerous larvae of the next stage (cercariae) that are released and are voided by the snail into the water. The cercariae can enter the human primary host through lesions in the skin, and traverse the bloodstream to the lungs and thence to the bladder. The complex life cycle confers advantages. The human host stage facilitates geographical dispersal of the parasite as humans migrate; the molluscan intermediate host or ‘vector’ stage allows rapid asexual reproduction of the larvae within the sporocysts. The intermediate host stage could be particularly advantageous for endoparasites where the problems for transfer from definitive host to definitive host are formidable.

In the malaria parasite, a sporozoan protoctistan, which lives in human red blood cells, blood-sucking mosquitoes, Anopheles spp., act as intermediate hosts or vectors. In Trypsanosoma brucei, which causes sleeping sickness, the zoöflagellate protoctistan lives in the cells of its human primary host and is transmitted by the intermediate host vector, the tsetse fly, Glossina spp.

Other parasites that utilize an indirect life cycle include the spiny-headed worms, the Acanthocephala (a minor pseudocoelomate phylum whose members are all parasites). The definitive hosts are usually craniates (vertebrates).

The parasites live in the gut, and shed their eggs in the faeces. Larvae hatch in the water and are ingested by crustacean intermediate hosts. Here they encyst and are released following the digestion of the arthropod vector subsequent to the ingestion of the vector by the definitive host. The platyhelminth tapeworms also have an indirect life cycle: the beef tapeworm Taenia saginata has a human definitive host and the cow as the intermediate host. In the Chinese liver-fluke Opisthorchis sinensis there are two intermediate hosts, a mollusc and a fish. Freshwater snails eat the eggs of this digenean trematode platyhelminth; the miracidia larvae that hatch in the snail initiate asexual reproduction. The resulting cercariae infect freshwater fish that are eaten raw by humans.

Most parasitic nematode roundworms have direct life cycles, but Wucheria bancrofti has an indirect cycle. Humans are the definitive host where Wucheria lives in the lymphatic system. The intermediate host, a mosquito, ingests juveniles in the human blood. The mosquitoes can, in turn, reinfect humans with a later developmental stage of the worm, following a mosquito bite.


The head louse Pediculus capitis that parasitizes humans is an example of an ectoparasite: it is a member of the insect order Anoplura (or Phthiraptera). Like most lice it is monoxenous. The larvae and the adults live in the hair. They have piercing mouthparts enabling them to pierce the skin and to suck blood. The legs are adapted by having clinging claws. The related body louse, Pediculus humanus, uses the same human host, but is ecologically and reproductively isolated from P. capitis. Lice pass from host to host by close contact. Although unpleasant, the louse does little harm; however, it can be dangerous because of hyperparasitism, whereby it carries parasitic rickettseae, tiny bacteria that cause typhus, ‘jail fever’. Many fleas (in the insect order Siphonaptera) are polyxenous; the rat flea Xenopsylla cheopis migrates from rodent to human hosts: the flea is in turn parasitized by the bacterium Yersinia pestis that causes septicemia and bubonic plague.

Parasite physiology and anatomy

Parasites tend to have adapted to the requirements of their environment and lifestyle. Co-evolution, whereby the host and the parasite evolve together seems to have been frequent. Often locomotion is modified, and there is a reduction in the nervous system, sensory organs (although often not chemoreception) and digestive systems. Guts are reduced and forms such as tapeworms are osmotrophs. Reproductive organs are frequently complex, associated with hermaphroditism or asexual/parthenogenetic reproduction. Parasites may have features that render them insensitive to the host’s immune systems or other defence mechanisms. Damage to the host can be through physical injuries, competitive exploitation of food resources or poisons.

Parasite control and eradication

The control and eradication of parasites is a subject beyond the scope of this article, but better public health measures, eradication of intermediate hosts or their habitats (with possible wider ecological side-effects), pharmacological agents to combat the parasites in vivo and vaccination programs may be relevant.