Disease is a harmful deviation from the normal structural and functional state of an organism manifested by signs and symptoms. It is usually associated with a particular abnormality in body tissues and may result from either physical injury, chemical injury or a change in the metabolism of living cells. It may be a consequence of a genetic mutation, adverse drug reactions, or environmental hazards. A disease is typically diagnosed by a physician or other health care professional with a history and physical examination. The identification of a disease is assisted by the use of laboratory testing and the review of medical records.
Diseases can be classified as infectious, non-infectious or idiopathic (of unknown cause). The term infectious diseases refers to any disease caused by one or more microorganisms that require contact with the human host and then overcome or bypass the innate immune defense mechanisms of the skin or internal mucosal surfaces of the respiratory, digestive or urinary tract. The infectious agents may be bacteria, viruses, protozoa or fungi.
Non-infectious diseases are generally those not caused by a communicable microorganism but rather by an acquired condition or abnormality in an organism such as cancer, heart disease, mental illness and some genetic diseases. A wide range of medications and surgical interventions are used in the treatment of non-infectious diseases.
The way we classify disease can have profound consequences for those who live with them. For example, conditions that were previously thought of as a normal part of ageing such as osteoporosis have now been recognized as diseases. Conversely, other physiological changes such as the decline in testosterone levels seen with ageing men are resisting attempts to be recognised as diseases. This raises questions about how and on what basis diseases are classified and in whose interests.
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