parasite

parasite Organism that lives on or in another organism (the host) upon which it depends for its survival; this arrangement may be harmful to the host. Parasites occur in many groups of plants and in virtually all major animal groups. A parasite that lives in the host is called an endoparasite; a parasite that survives on the host's exterior is an ectoparasite. Many parasites, such as protozoan, fleas and worms, carry disease or cause sores or lesions, which may become infected. The European cuckoo and cowbird rely on other birds to rear their young, and are therefore considered ‘brood parasites’. In parasitoidism, the relationship results in the death of the host. For example, various flying insects, such as the ichneumon flies, lay their eggs on or in a host that becomes the food for the insect larvae. A hyperparasite is one that parasitizes another parasite.

parasite (pa-ră-syt) n. any living thing that lives in or on another living organism (see host). Some parasites cause irritation and interfere with bodily functions; others destroy host tissues and release toxins into the body. Human parasites include fungi, bacteria, viruses, protozoa, and worms. See also commensal, symbiosis.
—parasitic (pa-ră-sit-ik) adj.

parasite one who obtains hospitality, etc. by obsequiousness XVI; animal or plant supported by another XVIII. — L. parasītus — Gr. parásītos one who eats at the table of another, toady, f. PARA-1 + sîtos food.
So parasitic XVII. Hence parasitical XVI.