Disease is a condition that results from disruption of the normal balance or homeostatic processes of an organism. In a basic sense, this imbalance may be caused by unfavourable exchanges with the organism’s external environment or by unfortunate evolutionary legacies. Disease may also arise from an infection transmitted from person to person or animal to animal, or by contamination of the body by chemical agents such as poisons, radioactive materials and heavy metals.
Generally, diseases have identifiable causes and can be diagnosed using tests. A disease can be contrasted with a disorder, which has no identifiable cause or may have a vague or indistinct one (such as mental illness). A syndrome is a group of symptoms that occur together and that do not have an obvious cause.
On a more practical level, disease is classified according to a catalogue that is maintained by international groups of experts for the World Health Organization, the International Classification of Diseases. Some conditions that were previously thought to be part of a person’s natural ageing process, such as osteoporosis or menopause, have been given disease status by this catalogue. This is sometimes justified, as in the case of osteoporosis, because it can lead to broken bones that threaten a person’s health.
Some diseases, such as HIV infection, will probably never be eradicated, but scientists continue to work towards cures by attempting to find ways to prevent and treat the disease before it develops into full-blown disease. Other diseases, such as cancer and heart disease, have been successfully treated to some extent.
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