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Types of classification
Alternatively, diseases may be classified according to the organ system involved, though this is often complicated since many diseases affect more than one organ.
A chief difficulty in nosology is that diseases often cannot be defined and classified clearly, especially when cause or pathogenesis are unknown. Thus diagnostic terms often only reflect a symptom or set of symptoms (syndrome).
Traditionally diseases were defined as syndromes by their symptoms. When more information is available, they are also defined by the damage they produce. When cause is known, they are better defined by their cause, though still important are their characteristics.
Probably the last described kind of diseases are molecular diseases, defined by their molecular characteristics. This was introduced in November 1949, with the seminal paper, "Sickle Cell Anemia, a Molecular Disease", in Science magazine, Linus Pauling, Harvey Itano and their collaborators laid the groundwork for establishing the field of molecular medicine.
Several classifications of diseases have been historically proposed, and normally all of them assign a code to every supported disease. Some of them codify diseases following the path of the classification tree, and others like SNOMED use a multifactor classification system.
The most known coding system is the World Health Organization ICD-Series, but there are other accepted classifications like DOCLE, NANDA or SNOMED. Historically there were others like the Berkson Coding System that are not maintained anymore.
There are also coding systems for symptoms present in the diseases and biological findings. They are normally included in medical dictionaries, also with a codification system. Some of them are MeSH (Medical Subject Headings), COSTART (Coding Symbols for Thesaurus of Adverse Reaction Terms) or MedDRA (Medical Dictionary for Regulatory Activities) Other systems like Current Procedural Terminology do not deal directly with diseases but with the related procedures.
Extended nosology and general medical conditions
Medical conditions, like diseases, can be defined by cause, pathogenesis (mechanism by which the disease is caused), or by a collection of symptoms, medical signs and biomarkers, particularly when the other two definitions are not available (idiopathic diseases).
From a nosological point of view, medical conditions could be divided in disorders, diseases, syndromes, lesions and injuries, each one with some specific meaning:
- In medicine, a disorder is a functional abnormality or disturbance. Medical disorders can be categorized into mental disorders, physical disorders, genetic disorders, emotional and behavioral disorders, and functional disorders. The term disorder is often considered more value-neutral and less stigmatizing than the terms disease or illness, and therefore is a preferred terminology in some circumstances. In mental health, the term mental disorder is used as a way of acknowledging the complex interaction of biological, social, and psychological factors in psychiatric conditions. However, the term disorder is also used in many other areas of medicine, primarily to identify physical disorders that are not caused by infectious organisms, such as metabolic disorders.
- The term disease broadly refers to any condition that impairs the normal functioning of the body. For this reason, diseases are associated with dysfunctioning of the body's normal homeostatic process. Commonly, the term disease is used to refer specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular organisms, and aberrant proteins known as prions. An infection that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal bacteria and yeasts in the gut, or of a passenger virus, is not considered a disease. By contrast, an infection that is asymptomatic during its incubation period, but expected to produce symptoms later, is usually considered a disease. Non-infectious diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease.
- Illness and sickness are generally used as synonyms for disease. However, these terms are occasionally used to refer specifically to the patient's personal experience of his or her disease. In this model, it is possible for a person to have a disease without being ill (to have an objectively definable, but asymptomatic, medical condition), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or medicalizes a non-disease situation in his or her life).
- Normally four main types of diseases are considered: pathogenic diseases, deficiency diseases, hereditary diseases, and physiological diseases.
- A syndrome is the association of several medical signs, symptoms, and or other characteristics that often occur together. Some syndromes, such as Down syndrome, have only one cause; others, such as Parkinsonian syndrome, have multiple possible causes. In other cases, the cause of the syndrome is unknown. A familiar syndrome name often remains in use even after an underlying cause has been found, or when there are a number of different possible primary causes.
- In cases of viral infections, like HIV, it is important to make a difference between the infection (considered as a disease, even while it is silent) and the associated symptoms (a syndrome). In the case of HIV the syndrome is named AIDS.
- Injury and lesions
- Injury is damage to the body. This may be caused by accidents, falls, hits, weapons, and other causes. Major trauma is injury that has the potential to cause prolonged disability or death. Lesion is any abnormality in the tissue of an organism (in layman's terms, "damage"), usually caused by disease or trauma. Lesion is derived from the Latin word laesio meaning injury. Similar to the ICD-10 the World Health Organization produces the International Classification of External Causes of Injury (ICECI). Sequelae of resolved diseases sometimes are considered inside lesions and other times inside diseases.
Some medical conditions cannot be classified in any of these groups, but they can still be important enough to be considered as medical conditions. For example, to be a carrier of a genetical disease, or a viral infection unable to progress to disease, normally is not considered inside any of the previous groups. Cases of infections able to progress, but with low possibilities, like latent tuberculosis, are also considered outside the category of diseases.
The term "medical condition" can also be applied to physiological states outside the context of disease, as for example when referring to "symptoms of pregnancy". It can also refer to the normal residual scars of a disease after it has resolved, for example lung fibrosis after a tuberculosis.
Ancient medical treatises had a variety of different ways of classifying and grouping illnesses. Chinese texts like the Huangdi Neijing categorized diseases by which of the atmospheric influences was believed to be responsible for them. Many ancient Greek, Mesopotamian, Roman, and Egyptian authors categorized diseases by the body parts they affected, while others divided diseases into acute or chronic illnesses. Mental disorders were classified into categories like mania and paranoia by Hippocrates, and this system was utilized by later authors like Najib ad-Din Samarqandi.
Many popular ancient disease classification systems largely relied upon humorism, which carried over into medieval times. Early attempts to develop more comprehensive approaches to the classification of diseases were made by Jean Fernel in the 16th century. Early modern nosological efforts grouped diseases by their symptoms, whereas modern systems focus on grouping diseases by the anatomy and cause involved.
In the 18th century, the taxonomist Carl Linnaeus, Francois Boissier de Sauvages, and psychiatrist Philippe Pinel developed an early classification of physical illnesses. In the late 19th century, Emil Kraepelin and then Jacques Bertillon developed their own nosologies. Bertillon's work, classifying causes of death, was a precursor of the modern code system, the International Classification of Diseases.
- Nosology is used extensively in public health, to allow epidemiological studies of public health issues. Analysis of death certificates requires nosological coding of causes of death.
- Nosological classifications are used in medical administration, such as filing of health insurance claims, and patient records.
- Clinical coder
- Diagnosis code
- Differential diagnosis
- International Statistical Classification of Diseases and Related Health Problems (ICD)
- ICD-10 (ICD 10th Revision)
- Medical classification
- Pathology (study of disease)
- Category:Diseases and disorders (Wikipedia's categorization of diseases)
- L Pauling, H Itano, SJ Singer, I Wells. "Sickle Cell Anemia, a Molecular Disease". Science, 25 November 1949, vol. 110, no. 2865, pp. 543-548.
- Abenhaim, Lucien et al. The Prognostic Consequences in the Making of the Initial Medical Diagnosis of Work-Related Back Injuries. Spine Journal, 1995
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- C. S. Herrman, The Bipolar Spectrum, SSRN (Social Science Research Network, 5 August 2010) 
- International Classification of Diseases by the World Health Organization.