From ArticleWorld

A parasite is an organism that needs to spend most of its life cycle living off the substance from a host organism in order to survive. Plants, animals and fungi can be either hosts or parasites. Almost all free-living animals become host to one or more parasites during their life time. Parasitism can be differentiated from symbiosis in that parasites derive benefit from the host but the host does not benefit—and may become ill—from the parasite.


Endoparasites are parasites that live within the bodies of the hosts. In humans, many parasites find their way through the intestinal tract and cause diseases such as Guardia, intestinal worms and Amebiasis. Other parasites affect the bloodstream and tissues. Malarial diseases affect the blood cells and are more difficult to treat than intestinal parasites.

Ectoparasites live on the outside of the host and do not generally cause as much systemic disease as endoparasites. Some types of leeches, lice, ticks and fleas are parasites that are visible on the outside of the body.

Parasites can be differentiated as necrotrophs, which are parasites that will continue to live off the host’s tissues until the host dies. They must have the ability to live temporarily on their own until another host is found. Biotrophs, on the other hand, can make the host feel ill but do not intend to kill the host so that, unless treatment is given, the host and parasite remain in a relatively static state.


Parasites that commonly affect humans are Giardia and Malaria, especially in areas where water is not clean or in warm climates. Giardia is passed via contaminated water and causes abdominal cramps and diarrhea that can persist for months until the diagnosis is made.

Malaria is passed via a mosquito vector and moves in the bloodstream to the liver where it incubates for several days before showing up in the red blood cells. When the red blood cells get full, they burst and release toxins and more of the malaria organism (Plasmodium species) that attack other red blood cells.

External parasites, such as lice, ticks and fleas have the ability to live for a brief time outside of a host until they must attach to a host and live off their skin or the blood underneath. Most of these kinds of parasites are annoying but not life-threatening.

Certain kinds of intestinal worms such as round worms, pin worms and tape worms generally infect only the worms; however, a worm known as Trichinella spiralis travels to the body tissue and harbors in the cells of the body. The disease, called Trichinosis can be severe.