Parasitism is a type of symbiotic relationship between organisms of different species in which one, the parasite, benefits from a prolonged, close association with the other, 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.Low Temperature Scanning Electron Microscope (LTSEM) image of the mite, Varroa destructor, on a honey bee hostAlthough 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 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. For example, because of the episodic nature of its feeding habits, the mosquito is not considered parasitic. 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