Nosography

Nosology (from Ancient Greek νόσος (nosos), meaning 'disease', and -λογία (-logia), meaning 'study of-') is a classification scheme within medicine delineating classification of diseases.

Diseases may be classified by cause, pathogenesis (mechanism by which the disease is caused), or by symptom(s).

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.

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