Throughout history, the use of illness as metaphor has existed in almost every mode of communication. Metaphors of sickness and disease have been used widely in writing, politics, and everyday speech to reference things considered bad or threatening. In novels and political discourse, the use of illness as metaphor is sometimes ongoing and thematic, with complicated literary or rhetorical dimensions. In other types of language, sickness or disease is used metaphorically for effect by means of comparisons, similes, and figures of speech.
The different uses of illness as metaphor go hand in hand with the human tendency to compare things to the human body. Analogies to the human body occur everywhere in language, and they include broad metaphors such as the body of a political system, the brains of an organization, or the heart of a business. Anything that is expected to function in a certain way can be said to be healthy or unhealthy, and particular physical and mental ailments become effective metaphors for describing situations when things aren't functioning the way they should, or when things aren't functioning as someone perceives that they should.
Simple uses of illness as metaphor include applications to things broken or in poor condition. Machines and technology are often described in terms of their health: for example, an unhealthy-sounding engine or a sick computer with a virus. Grass, trees, and fields are similarly described: for example, an unhealthy lawn, a sick tree, or a diseased crop. This might extend to neighborhoods or parts of a country. Likewise, interpersonal, social, economic, and political relationships are all frequently referenced in terms of good or poor health: for example, an unhealthy marriage, a sick society, an ailing business, or a diseased justice system.
Specific illnesses are also used as analogies in arguments and descriptive language. Mental illnesses like schizophrenia, ADD (attention deficit disorder), and Alzheimer's disease are often used as figures of speech for anything that exhibits qualities of disorganization, lack of focus, or forgetfulness. Obesity is sometimes applied to excess or surfeit, while anorexia to extreme thinning out, denial, or parsimony. Cancer is used rhetorically, especially in political arguments, to describe anything dangerous, pernicious, or insidious. This would include things that are very bad, hard to see, and operating with a hidden, self-serving, or malicious agenda.
In literature and philosophy, authors have extensively used illness as metaphor to describe religious, social, and political ills. The 19th-century Symbolist and Decadent movements frequently used metal illness as an analog for artistic sensibility. Various modern schools of literature and philosophy, such as Existentialism, made frequent analogies between sickness and spiritual, social, and political corruption or malaise. In the late twentieth century, extensive scholarly attention was given to the many literary uses of sickness and disease, making illness and pathology a popular trope for literary criticism and theory.