Parasitism is a type of symbiotic relationship between organisms of different species Fact: date=September 2008. The parasite benefits from a prolonged, close association with the host, which is harmed. In general, parasites are much smaller than their hosts, show a high degree of specialization for their mode of life and reproduce more quickly and in greater numbers than their hosts. Classic examples of parasitism include the interactions between vertebrate hosts and such diverse animals as the tapeworms, flukes, Plasmodium species and fleas.

The harm and benefit in parasitic interactions concern the biological fitness of the organisms involved. Parasites reduce host fitness in many ways, ranging from general or specialized pathology (such as castration), impairment of secondary sex characteristics, to the modification of host behaviour. Parasites increase their fitness by exploiting hosts for food, habitat and dispersal.

Although the concept of parasitism applies unambiguously to many cases in nature, it is best considered part of a continuum of types of interactions between species, rather than an exclusive category. Particular interactions between species may satisfy some but not all parts of the definition. In many cases, it is difficult to demonstrate that the host is harmed. In others, there may be no apparent specialization on the part of the parasite, or the interaction between the organisms may be short-lived. In medicine, only eukaryotic organisms are considered parasites, to the exclusion of bacteria and viruses. Some branches of biology, however, do regard members of these groups to be parasitic.Fact: date=February 2008

Types of parasitism

Those that live inside the host are called endoparasites (e.g., hookworms) and those that live on its surface are called ectoparasites (e.g., some mites).

An epiparasite is one that feeds on another parasite. This relationship is also sometimes referred to as "hyperparasitism".

Parasitoids are organisms that cause the host to die as a result of parasitism. Thus, the interaction between the parasitoid and the host is fundamentally different than true parasites and their host, and shares some characteristics with predation. Social parasites take advantage of interactions between members of social organisms such as ants or termites. In kleptoparasitism, parasites appropriate food gathered by the host. An example is the brood parasitism practiced by many species of cuckoo. Many cuckoos use other bird species as "babysitters", depositing their eggs in the nest of the host species, which raise the cuckoo young as one of their own.