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A parasitic relationship is a relationship between organisms in which one organism, the parasite, gains some benefit from the other organism while the other organism, the parasite host, is harmed by the relationship. The parasite host is usually much larger than the parasite. Tapeworms, for example, are parasites that reside in the intestines of some vertebrates. While they can get very long, their bodies generally remain small enough to fit in the host's intestinal tract.
Parasites use their hosts for many different purposes. Most commonly, parasite hosts are used as a food source for the parasite. They are also often used as a habitat for the parasites. Hookworms, for example, take residence in the intestinal tracts of mammals such as dogs, cats, and humans. Finally, parasites use their hosts as mediums for reproduction; tapeworms and other parasites tend to reproduce in great numbers inside their hosts.
In some parasitic relationships, the larval development of the parasitic organism occurs in the parasite host's body. Parasites that do this are called parasitoids. In such situations, the parasite host is almost invariably killed. In many cases, the parasitoid consumes the host as well.
There are many different parasitic relationships that a parasite host can be in. In kleptoparasitism, the parasite steals food gathered by the host. Brood parasites lay eggs in the nests of other organisms, who serve as surrogate parents and keep the eggs safe. Often, the parasite removes one of the original eggs to make room for its own.
Generally speaking, only relationships between complex organisms can be considered parasitic relationships. For some purposes in biology, however, viruses are also considered parasites. Viruses attack and invade host cells, use them to reproduce, and spread to more cells, destroying the parasite host cell. Viruses, however, are generally not considered to be living organisms as they are not made up of cells
Parasitology is a branch of microbiology that deals with relationships between parasites and their hosts. The field focuses on how parasites spread, what they gain from their hosts, and how they harm their hosts. Parasitologists often assist medical professionals in curing parasitic infections in humans.
It is not uncommon for a human to be a parasite host. Parasites infect humans through insect bites, uncooked meat, contaminated vegetables, and dust containing parasitic larvae. Many different types of worms, such as flatworms and liver flukes, commonly infect humans. Many types of tiny protozoa can also take residence in the human body.