Disease is any disturbance in the structure or function of the body or any of its parts, an imbalance between the individual and their environment, or a lack of perfect health. The problem with this definition is that it could include activities such as clipping toenails or being tied to a chair as diseases.

The difficulty lies in finding a definition that is neither overly broad, too narrow, nor too vague. Perhaps the problem stems from society itself. We do not even agree on what constitutes a disease. For instance, are baldness, infertility, or hypochondria considered diseases? Are penile dysfunction or premature ejaculation medical conditions for which drugs should be available, or do they simply reflect inappropriate societal expectations about sex? Is cosmetic surgery a medical treatment or a lifestyle choice?

If a person eats very little and becomes thin, they may be labeled as anorexic and presumed to lack mental capacity; if someone eats a lot and becomes obese, many people regard it as a moral failing. Additionally, someone who is considered unattractive may face disadvantages due to their appearance—does that make ugliness an illness?

These examples highlight the complex interplay between societal expectations, moral values, and illness. The question arises: is the issue with the individual's body or with societal expectations about bodies?

A common view is that ill-health represents a departure from what is considered 'normal' for a human being. However, this approach assumes a clear definition of what is normal. Biology does not provide a definitive answer to what is normal. Robin Mackenzie has argued that:

"Medical conceptions of normal health have shifted from a statistical mean of characteristics and capacities to an elevated, more ideal level of health, attainable for most of us only through taxing regimens of discipline and self-denial. Anything less, although statistically more representative, becomes pathologized as ill health."

It is evident that social factors influence what is perceived as illness. Homosexuality was regarded as a psychological illness by the American Psychiatric Association until 1973, and only in recent times have dyslexia and myalgic encephalomyelitis (ME) been widely accepted as medical conditions.

Anyone attempting to define disease or ill-health must consider these issues.