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).
One of the earliest efforts at developing a classification of diseases began in the 10th century, when the Arabian psychologist Najab ud-din Unhammad classified a nosology of nine major categories of mental disorders, which included 30 different mental illnesses in total. Some of the categories he described included obsessive-compulsive disorders, delusional disorders, degenerative diseases, involutional melancholia, and states of abnormal excitement.
In the 18th century, the taxonomist Carolus Linnaeus, Francois Boissier de Sauvages, and psychiatrist Phillipe Pinel developed an early classification of physical illnesses. 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 code system International Classification of Diseases.