A disease is an alteration of the health or functions of living organisms (animals or plants), particularly when the cause is known as an aggression by a harmful organism on a host organism. This alteration (or disorder) has visible signs called symptoms.

Disease

A condition which alters or interferes with a normal process, state, or activity of an organism. It is usually characterized by the abnormal functioning of one or more of the host's systems, parts, or organs. Included here is a complex of symptoms descriptive of a disorder. Source UMLS