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From the latin parasītus (but whose origin is more distant, from a Greek word meaning "eaters"), a parasite is an organism that lives at the expense of another species. The parasite, which can be of animal or vegetable origin, feeds the other body while the weakening but without actually killing most of the time.
The biological interaction involving parasites is called parasitism. The species that hosts the parasite is the host. He suffered a deprivation of their reproductive capacity under the action of the other agency which, in turn, manages to improve its own conditions and its ability to survive.
We can say, therefore, that parasites have the association they establish with other body, while it is hampered due to this interaction.
In some cases, parasites themselves can become the hosts of a third species, namely, the hyperparasites. Thus, a string occurs in which the hyperparasite lives at the expense of the parasite, and the parasite did the same with its host.
The wire of time and successive generations, host organizations are able to develop some mechanisms of defense that away pests or minimize their field of action. However, parasites can undergo morphological and physiological changes through natural selection.
Finally, in the familiar, a parasite language said any person who lives or who tries to live at the expense of others while enjoying their hardware resources. For example: "Maximilien is a parasite that has never worked. If it is not his parents, it would be by train to sleep in the street".