Parasitism

**Parasitism** is a form of one-sided symbiosis. The parasites live off the . They may, or may not, harm the host.  Parasitoids, on the other hand, usually kill their hosts. A parasitic relationship is the opposite of a mutualistic relationship. Examples of parasites in humans include tapeworms and leeches. World-wide, the most serious cause of human death by a parasite is malaria.

A definition:
* A parasite is an organism drawing nutrients from a living host. It lives in or on another organism, getting from it part or all of its food. It usually shows some degree of adaptive modification, and causes some degree of damage to its host.

## Parasites on humans 
"Humans are hosts to nearly 300 species of parasitic worms and over 70 species of protozoa, some derived from our primate ancestors and some acquired from the animals we have domesticated or come in contact with during our relatively short history on Earth. Our knowledge of parasitic infections extends into antiquity".

## Biological context 
When the above definition is applied, many organisms which eat plants can be seen as parasites, because they feed largely or wholly on one individual plant. Examples would include many herbivorous insects: the Hemiptera or true bugs (leafhoppers, froghoppers, aphids, scale insects and whiteflies). The larvae of Lepidoptera usually feed and mature on a single individual of the host plant species, and what they eat accounts for most of the food for their complete life span. Moreover, caterpillars can and often do serious damage to the host's foliage. Other orders also have many parasitic herbivores: Thysanoptera (thrips), Coleoptera (beetles), Diptera (flies).
s parasitising a harvestman]]

Parasites of larger animals account for much research done for veterinarian and medical purposes. These parasites include viruses, bacteria, protozoa, flatworms (flukes and tapeworms), nematodes (roundworms), arthropods (crustacea, insects, mites). Parasitic wasps and flies are of great interest to the entomologist, and may be used in biological control.

On the other hand, many blood-sucking insects (such as mosquitoes) have only brief contact with a host, and so perhaps should not be regarded as parasites.p5

A huge number of species are parasitic. A survey of the feeding habits of British insects showed that about 35% were parasites on plants, and slightly more were parasites on animals. That means that nearly 71% of insects in Britain are parasitic. Since British insects are better known than those elsewhere (because of the length of time they have been studied), this means that the majority of insect species throughout the world are parasitic. Also, there are several other invertebrate phyla which are wholly or largely parasitic. Flatworms and roundworms are found in virtually every wild species of vertebrate. Protozoan parasites are also ubiquitous. Hence parasitism is almost certainly the most common feeding method on Earth.
:"It is clear that parasitism as a way of life is more common than all other feeding strategies combined". For a small organism, the distances between hosts, or groups of hosts, is a hazard. Adaptations to bridge this hazard are:
* Mass production of spores or eggs.
* Dispersal of fertile females.
* Dispersal by attaching to a larger organism (many ectoparasites).
* Extreme longevity of resting stages (spores or cysts). For example, plant parasitic nematodes may remain quiescent for 23 years.
* Parthenogenesis, and inbreeding among offspring of a single female, are common in parasites.

So parasites exist in small, genetically similar groups with little flow of genes between them. In consequence, they have adaptations to solve their problems of dispersal and reproduction.
:1. The pathogen bacterium Rickettsia.
:2. Chiggers, which are ectoparasitic mites.
:3. Small mammals, especially rats.
:4. Scrubby vegetation with moist conditions
:5. Humans passing through patches where the above are present.

The occurrence of all these factors together would be limited in space, and brief in time. This is typical of the ecology of parasitic infections.

## Evolution 
### Rapid speciation 
Both evolutionary rates and speciation rates can be high. Sibling species are very common in the bug *Erythroneura*, in which about 150 transfers from one host to another has resulted in about 500 species in the genus.

The clearest evidence comes from the large size of many parasitic families. 
:"Even though some parasitic taxa evolved much later than predatory taxa, families of parasites on plants are on average almost eight times larger than those of predators, and families of parasites on animals are over ten times larger".

### Adaptive radiation 
Adaptive radiation in parasites is extensive. Its development in each taxon (group) depends on:
:The diversity of the hosts. How many species there are in the group being parastised, and how different they are from each other.
:The size of the host target: body size, population, geographical distribution.
:The evolutionary time available for colonisation of hosts.
:The selective pressure for evolutionary modification.

Diversity of hosts is a big factor. If many related species of host are available, then many related species of parasite will evolve. Mites on Lepidoptera families or fleas on mammals and birds are good examples. Eichler's rule goes as follows:
:"When a large group of hosts consisting of a wide variety of species is compared with an equivalent group consisting of few representatives, *the larger group has the greater diversity of parasitic fauna*".

The two British oaks support some 439 species of parasite directly, and indirectly many hundred more which parasitise these parasites. "It would certainly be an underestimate to say that the two British species of oak are the primary products for a thousand species of parasite".

## Further reading 
*Schmidt-Hempel 2011. *Evolutionary parasitism: the integrated study of infections, immunology, ecology, and genetics*. Oxford University Press.  

## References