Disease is any measurable and sustained deterioration of health that has been associated with dysfunctions in the normal homeostatic control systems of the body. It can be caused by many different things and can be divided into a number of categories, such as infectious diseases, genetic diseases and disorders, metabolic-endocrine diseases and cancers.
Some diseases are curable, while others may progress or recur over long periods of time, such as slow infections (such as Lyme disease) or degenerative neurological diseases like Parkinson’s and Alzheimer’s disease. Diseases that have no cure are known as incurable and usually cause chronic symptoms with no hope of a full recovery.
Diseases are generally indicated by signs and symptoms, with the term sign being a physical evidence of the condition verified by a physician or other healthcare professional, and symptom being an individual’s own perception of their discomfort or illness. There is often a significant discordance between a patient’s conception of a disease, and its objective manifestation as recorded by a medical diagnosis.
Often, the definition of what constitutes a disease changes over time, for both social and medical reasons. For instance, osteoporosis has gone from being a natural part of the ageing process to being classed as a disease. In other cases, risk factors such as high blood pressure are reclassified as diseases, resulting in more people being diagnosed and treated for this condition. This is partly due to changing expectations of the state of good health, but also because a change in definition will make it easier to qualify for treatments under healthcare insurance schemes.