Nosology

Nosology (from the Greek "nosos," "disease") is a branch of medicine that deals with classification of diseases.

Diseases may be classified by etiology (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 etiology or pathogenesis are unknown. Thus diagnostic terms often only reflect a symptom or set of symptoms (syndrome).

Some of the earliest efforts at developing a classification of diseases began in the 18th century with the taxonomist Carolus Linnaeus and Francois Boissier de Sauvages, although Thomas Sydenham's work in the late 17th century might also be considered a nosology.

In the 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 medical-billing code system, ICD.

The early nosological efforts grouped diseases by their symptoms, whereas modern systems (e.g. SNOMED) focus on grouping diseases by the anatomy and etiology involved.