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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.
Syndromes, illness and disease
Medical usage sometimes distinguishes a disease, which has a known specific cause or causes (called its etiology), from a syndrome, which is a collection of signs or symptoms that occur together. However, many conditions have been identified, yet continue to be referred to as "syndromes". Furthermore, numerous conditions of unknown etiology are referred to as "diseases" in many contexts.
Illness, although often used to mean disease, can also refer to a person's perception of their health, regardless of whether they in fact have a disease. A person without any disease may feel unhealthy and believe he has an illness. Another person may feel healthy and believe he does not have an illness even though he may have dangerously high blood pressure which may lead to a fatal heart attack or stroke.
Identifying a condition as a disease
Identifying a condition as a disease, rather than simply a variation of human structure or function, could have significant social or economic implications (such as compensation for the victims). For example, recognition of post-traumatic stress disorder, also known as "shell shock"; repetitive motion injury or repetitive stress injury (RSI); and Gulf War syndrome as diseases were very controversial in several countries, and affect financial and other responsibilities of governments and companies to individuals. Also, ageing is increasingly being viewed as a disease, although this is widely disputed.
A condition may be considered to be a disease in some cultures or eras but not in others. To consider a syndrome to be a disease is a social value judgement. Oppositional-defiant disorder , attention-deficit hyperactivity disorder, and, increasingly, obesity are conditions considered to be diseases in North America today, but not some decades ago, and not in some other countries. Conversely, the number of people in the West who consider homosexuality to be a disease has been decreasing.
Transmission of disease
Some diseases, such as influenza, are contagious or infectious, and can be transmitted by any of a variety of mechanisms, including droplets from coughs and sneezes, by bites of insects or other vectors, from contaminated water or food, etc.
Other diseases, such as cancer and heart disease are not considered to be due to infection, although micro-organisms may play a role.
Other uses of the term
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.