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A disease is any abnormal condition of the body or mind that causes discomfort, dysfunction, or distress to the person affected or those in contact with the person. Sometimes the term is used broadly to include injuries, disabilities, syndromes, symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts these may be considered distinguishable categories.

Pathology is the study of diseases. The subject of systematic classification of diseases is referred to as nosology. The broader body of knowledge about diseases and their treatments is medicine.

Older medical usage sometimes distinguished a disease, which has a known specific cause or causes (called its etiology), from a syndrome, which is a collection of signs and/or symptoms that occur together. This pedantic distinction has become even less valid as the causes of many syndromes have been identified. Also, many medical terms that describe symptoms or abnormalities may be referred to as "diseases" in many contexts, especially when the cause of the problem is unknown.

A condition can be objectively verifiable, but considering it a disease is a social value judgement. For example, in current North American society the number of people considering shortness and obesity as diseases to be treated has been increasing over the last 40 years, and the number of people who consider homosexuality to be a disease has been decreasing.

A condition may be considered a disease in some cultures or eras but not in others. Oppositional-defiant disorder, attention-deficit hyperactivity disorder, and sociopathic personality disorder are examples of conditions considered diseases in current North American society but not recognized in American culture a century ago or in many other cultures currently.

Sometimes whether a condition should be considered a disease or a variation of human structure or function becomes an intensely political controversy because of significant social or economic implications. For example recognition of post-traumatic stress disorder, also known as shell shock, were highly politicized processes in the United States, as was repetitive motion injury in Australia.

One of the largest and best-known categories of disease, infectious diseases are those caused by transmissible infectious agents such as bacteria, fungi, parasites, viruses, and prions.  Closely related though not infectious diseases in the strictest sense are parasitic diseases caused by protozoa and worms.  There are also genetic diseases caused by the presence or absence of genes in the affected person's DNA; toxic diseases caused by exposure to environmental toxins such as heavy metals; nutritional diseasess caused by lack or deficiency in certain nutrients; conditions caused by injury, malformation, or disuse of parts of the body; autoimmune diseases caused by immune system attacks on the body's own tissue; diseases caused by the patient's own beliefs; and diseases caused by combinations of these, and of course totally unknown causes.

The World Health Organization publishes a comprehensive list of diseases known as International Statistical Classification of Diseases and Related Health Problems (ICD). It was originally designed as a tool for describing diseases from a public health perspective. In the United States the ICD9 code list has been primarily used for insurance and billing purposes and is widely considered obsolete and incomplete.

See also:

In biology, disease refers to any abnormal condition of an organism that impairs function.

The term "disease" is often used metaphorically for disordered, dysfunctional, or distressing conditions of other things, as in disease of society.

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