Parasite

From Palaeos

A parasite is an organism that derives nutrition from other living organisms, generally in such a way as to cause disadvantage but not death to the victim. The term also usually implies a long-term association, and/or that the "parasite" is smaller than the "host". Herbivores are excluded, even though they would often fall under the coverage of the first definition (cattle are not regarded as parasites of grass).

Parasites are generally divided between endoparasites, which live within the host, and ectoparasites, which live outside the host. Endoparasites may be further divided according to where exactly they can be found, whether in the gut, intracellularly, etc. Ectoparasites may differ between those that are permanently associated with the host (such as lice) or those that only approach the host to feed (such as mosquitoes). The adaptations associated with all these categories can differ significantly. Many parasites, particularly endoparasites, show a reduction or loss of features found in their free-living relatives, and rely on the host for the functions these features previously performed. Many parasites are often very host-specific, and many endoparasites have complicated life-cycles with different stages having different hosts.

Because most parasites are soft-bodied and have restricted habitats, most parasitic clades have very little fossil record.

A great many different organisms have become parasites varying from bacteria to some vertebrates such as vampire bats, and it would not be possible to enumerate them all. However, some notable examples are listed below:

Bacteria: Many bacteria are known for causing diseases, such as Mycobacterium tuberculosis (tuberculosis) and Treponema pallidum (syphilis). Many bacteria and fungi are facultative parasites - that is, they normally live on the host as commensals and only become parasitic if the host has been otherwise compromised.

Protozoa: "Protozoa" include many parasitic clades such as Diplomonadida, Kinetoplastida and Sporozoa, some members of which are significant disease organisms for humans.

Embryophyta: A number of plants are parasitic, generally on other plants. Plant parasites may be distinguished between hemiparasites such as Loranthaceae, which retain their own leaves and only derive some of their nutrition from the host, and holoparasites such as Rafflesiales, that derive their nutrition entirely from the host.

Fungi: Notable examples of parasitic fungi are the Microsporidia (intracellular parasites only recently recognised as fungi) and members of Urediniomycetes (the rusts).

Animalia: Among the countless examples of parasitic animals are the Myxozoa, Mesozoa, parasitic flatworms such as flukes and tapeworms, ns such as many copepods, and insects such as lice and fleas. Many wasps and flies lay eggs within other insects - technically, these insects are parasitoids, as unlike true parasites the larva usually causes the death of the host.