Parasitism is a symbiotic relationship in which one organism benefits (the parasite) and the other organism is harmed (the host).
Often parasitism involves the parasite obtaining nutrition directly from the host organism. Prominent biologist, E.O. Wilson, has likened parasitism to predation in which, "...the predator consumes the prey in units of less than one." For instance, the fungi, Tinea pedis, causes the symptoms of athlete's foot in humans because it is actually consuming the tissue around the foot. However, parasitism does not always involve one organism directly consuming the other. In many cases, such as with tapeworms, the parasite simply "steals" some of the food that is being digested by the host, and, among parasitic plant species, the strangler fig of the topical rainforest doesn't consume any nutrition obtained by its host: it simply out-competes its host for light.
Like many symbioses, parasitic relationships are typically very host-parasite specific: in some cases even to the level of the species. Malaria is a wonderful example of the incredibly complex and specific nature of many host-parasite relationships. While there are similar diseases that affecting other species, the cause of the disease in humans, several species of protists from the genus Plasmodium, are, with perhaps one exception, entirely specific to their human and the Anopheles mosquito hosts.
At this point, it would be appropriate to consider a few aspects of parasitism. In some parasitic relationships, the host organism is minimally harmed. How many times can a person be bitten by mosquitos before they die? Similarly, Plasmodium does not consume its host immediately, although, if left untreated, malaria can kill a human in a relatively short period of time. For what reason do most parasites only weaken or very slowly kill their host? The answer should be apparent by thinking of the complexities of the Plasmodium life cycle outlined above. If infected humans were to die before other mosquitos had taken in a blood meal loaded with gametocytes, the parasite would quickly become extinct. In this case, the parasite has adapted to killing its host relatively slowly and only incidentally. After all, wouldn't it be better for the parasite if the host could live indefinitely?
Parasitism, like all symbiotic interactions, demonstrates the complex nature of the biosphere. As humans we tend to think of ecosystems as existing outside of the bodies of the organisms that fill them, but the fact is that the ecosystem extends deeply within the large and small biotic units of which it is composed. At every level where there is energy to be obtained, organisms, most of them microscopic, are found. The extent of biodiversity is such that even the parasites have parasites!
**Button Icons from www.grsites.com**
**Map obtained from: www.wehi.edu.au/MalDB-www/intro.html
**Anopholes mosquito: http://www.sun-city-south-africa.com/images/sun-city/mosquito.jpg
**Plasmodium vivax life-cycle: www.malaria.freewire.co.uk/prevent.htm
**Plasmodium sporozoites: www.anaesthetist.com/icu/infect.malpix.htm
**Plasmodium schizont and oocysts: www.life.sci.qut.edu.au/LIFESCI/darben/protozoa.htm
**Plasmodium merozoite: www.bucconeer.worldcon.org/contest/2002e_f6.htm
**Plasmodium merozoites rupturing red blood cells: navymedicine.med.navy.mil/med26/news/news-rel/news2000.htm