Plants or fish cultivated for decorative rather than functional purposes.  

Invertebrate organisms that live on or in another organism (the host), and benefit at the expense of the other. Traditionally excluded from definition of parasites are pathogenic bacteria; fungi; viruses; and plants; though they may live parasitically. [NALT]

An organism that lives at the expense of another (host) during some portion of its life cycle. Usually a parasite does not kill its host. [AGROVOC]

Microorganisms, viruses and parasites that can cause disease.

A general term for organisms which may cause illness or damage to humans or to crops, livestock, or materials important to humans. [NALT]

An animal, plant, fungus or other organism that thrives in places where it is not wanted by people, for example in fields, with livestock, in forests or in gardens. [AGROVOC]

Disease Vectors: Invertebrates or non-human vertebrates which transmit infective organisms from one host to another.