Parasite

Parasite, organism that lives in or on a second organism, called a host, usually causing it some harm. A parasite is generally smaller than the host and of a different species. Parasites are dependent on the host for some or all of their nourishment. For example, a tapeworm, a flattened worm that lives in the gastrointestinal tract of mammals, lacks an intestine of its own and must absorb predigested food from the intestine of its host. This food is the tapeworm’s only energy source for growth and reproduction. Parasitism affects most life forms, from bacteria infected by the viruses known as bacteriophages, to humans, who are subject to more than 100 parasites known to cause disease.
Parasites come in a variety of forms. Many arthropod parasites, including mites, ticks, and mosquitoes, cause a number of debilitating animal and human diseases. Certain plants, including mistletoe and dodder, parasitize other plants to obtain water and nutrients. Microscopic parasites include single-celled protozoans such as amoebas and sporozoa, fungi, and bacteria, which can infect animals and plants. Viruses are entirely parasitic, able to survive and reproduce only within other living organisms.
Parasites that live on the inside of the host’s body are known as endoparasites, while those that live on the outer surface of their hosts are known as ectoparasites. This distinction reflects adaptations made by the parasite to overcome certain barriers to parasitism. For example, when invaded by a parasite, a host often triggers an immune response, a cellular reaction that works to destroy the invader. Parasitic worms, including flatworms (soft-bodied worms, such as tapeworms and flukes) and roundworms (thin, unsegmented worms, also called nematodes) are endoparasites, usually living in the intestines, lungs, liver, or other internal organs of their hosts. These worms have developed adaptations that enable them to avoid the host’s immune response, such as during a developmental stage when they are protected by a cyst wall or an outer surface that constantly changes, thereby making it difficult for the host immune system to target the parasite for attack.
Many ectoparasites have developed structures, such as suckers, hooks, and teeth, which help penetrate the host’s outer surface. Primitive fishes, such as hagfish and lampreys, use suctionlike mouths to attach to the outer surface of other fish and suck out nutrients. Some annelids (segmented worms), such as leeches, are also ectoparasites, using sucking disks to feed on the blood and tissues of vertebrate hosts