parasitism

n. living at the expense of or off the body of another organism; manner of living in which an organism obtains its livelihood at the expense of another creature

Parasitism is one version of symbiosis ("living together"), a phenomenon in which two organisms which are phylogenetically unrelated co-exist over a prolonged period of time, usually the lifetime of one of the individuals. The requirement for a prolonged interaction precludes predatory or episodic interactions (such as a mosquito feeding on a host), which are usually not seen as symbiotic relationships. Symbiosis encompasses commensalism ("eating at the same table", wherein two organisms co-exist in the same space, and one organism benefits while neither harming nor helping the other), through mutualism (wherein both species benefit from the interaction) to parasitism, wherein one organism, usually physically smaller of the two (the parasite) benefits and the other (the host) is harmed. (Various forms of "social parasitism", kleptoparasitism, and "cheating parasitism", as discussed below, are characterized by a less close association between a parasite and a host, however.) Especially in the field of medical parasitology, the term "parasite" has come to mean a eukaryotic, pathogenic organism. Thus, protozoan and metazoan infectious agents are classified as parasites while bacteria and viruses are not. Interestingly, fungi are not discussed in textbooks of medical parasitology, even though they are eukaryotic.
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Parasitism /par·a·sit·ism/ 1. symbiosis in which one population adversely affects another, but cannot live without it. 2. [more]Parasitism - Community and Resources

Noun
1. the relation between two different kinds of organisms in which one receives benefits from the other by causing damage to it (usually not fatal damage)
(hypernym) mutuality, interdependence, interdependency

(n.)
The state or behavior of a parasite; the act of a parasite.
  

(n.)
The state of being parasitic.