Parasites are organisms living in or on another of a different kind, deriving their subsistence form the living material, digested food, secretions or other products of their host, are inextricably bound up with their host or hosts in the continuance of their life, and is rather injurious than beneficial in their influences. Parasitism is a relation of dependence, always nutritive, often more, between the parasite and the host but the inter-relation takes so many forms that absolutely precise definition is impossible, and it is not easy to separate off parasitism from other vital associations. A living creature habitually growing on a plant is called an epiphyte; it is not a parasite unless it gets its food in whole or in part from its bearer, as mistletoe does. A living creature growing on an animal is called a epizoic, like a barnacle on a whale; it is not a parasite unless it gets its food in whole or in part from its bearer, such as many fish lice. Parasitism is not symbiosis, a mutually beneficial internal partnership between two organisms of different kinds, such as unicellular algae which live within Radiolarians, some polyps, and a few worms. Nor is it commensalism, a mutually beneficial external partnership between two organism of different kinds, such as some kinds of hermit crabs which are always accompanied by sea-anemones. The distinction between parasitism and these other forms of relations is difficult to make. A distinction between parasites and predators must be made because killing off the host is not in the interest of the parasite.
Parasitism is also a term used metaphorically to describe the poor and the rich, depending on who is using it, and other human relationships.