Disease is any harmful deviation from the normal structural or functional state of an organism with characteristic signs and symptoms. The concept of disease has profound implications for medicine, health care and public policy. It is one of the most fundamental and complex concepts in medicine and a central theme in the philosophy of science. The study of diseases is called pathology and includes the determination of etiology (cause) of disease, its progression, and the morphological changes associated with disease.
A disease can be caused by a variety of microorganisms including viruses, bacteria, fungi, protozoa and multicellular organisms. It can also be the result of an abnormal inherited genetic mutation or a side effect from a drug treatment. Diseases can also be divided into categories such as infectious, hereditary, drug induced and autoimmune disease.
Many diseases are non-infectious and progressive. Examples include cancer, heart disease and chronic diseases such as diabetes. They can be further classified as non-communicable, rare and systemic diseases.
While it seems obvious that a disease is anything that causes harm, it can be difficult to define. This is especially the case for conditions that are psychological or socio-economic in origin. For example, many risk factors of diseases such as high blood pressure eventually get redefined as a disease in their own right – but it’s not so easy to decide whether this is in the best interests of the patients and society.