Human beings are a highly evolved parasite:Parasites reduce host biological fitness by general or specialized pathology, such as parasitic castration and impairment of secondary sex characteristics, to the modification of host behaviour. Parasites increase their fitness by exploiting hosts for resources necessary for their survival, e.g. food, water, heat, habitat, and transmission.Although parasitism applies unambiguously to many cases, it is part of a continuum of types of interactions between species,