A parasite is an organism which exploits another organism for the purpose of staying alive. Some parasitic relationships are harmless, while in other cases a parasite can damage or even kill its host. The study of parasitism is an extensive field, because parasites can be found across the biological kingdoms, and many animals host one or more parasites during their lives. A number of organisms also go through a parasitic stage at some point during their lives.
The word is borrowed from the Greek parasitos, meaning “one who eats at the table of someone else.” In both Greece and Rome, some people made dining at the homes of others a full time occupation, sometimes being called “professional dinner guests.” Like biological parasites, these individuals exploited their hosts for food, and they brought nothing to the the table themselves, other than dinner conversation. The existence of parasites has long been known in biology, although the development of high quality microscopes greatly expanded human knowledge of parasites.
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In order to be considered a parasite, an organism must depend on another for food, energy, or some other service, such as incubating and raising young. In addition, the parasite must bring nothing to the relationship, creating an arrangement which may be neutral or harmful, but never positive. Numerous organisms team up together to exploit their mutual strengths in a biological process called symbiosis—in this case, the arrangement is mutually beneficial to both creatures and it is not considered parasitism.
Some well known examples of parasites include mites, tapeworms, mistletoe, and fleas. Parasites live in a number of different ways; some, for example, cannot live once their host dies, while others can switch hosts or continue thriving on dead hosts until their nutrients are consumed. There is some dispute over whether bacteria and viruses should be considered parasites; in medical terms, a parasite is usually a eukaryotic organism, meaning that it has a complex cell structure, unlike a bacterium.
Parasites which live inside a host are called endoparasites or internal parasites. Many human diseases are caused by internal parasites, which may infest the intestinal tract, causing symptoms like diarrhea and vomiting. Various treatments are used for parasitic infection, depending on the organism involved. Ectoparasites live outside the host, and they are generally more capable of switching hosts. When a parasite preys on other parasites, it is known as an epiparasite.