Nosology

Nosology (in Greek Nosos = Disease) is a branch of medicine that deals with classification of diseases.
Diseases can be classified by etiology (cause), pathogenesis (mechanism in which the agent causes disease), or by symptom. Alternatively, diseases can be classified according to the organ system they involve, but this is often complicated as many diseases affect multiple organs.
One of the main problems with nosologies is that diseases often cannot be defined and classified clearly especially when pathogenesis or causality is not known. So, diagnostic terms often are in fact only symptoms or sets of symptoms (syndromes). 
==See also==
*Differential diagnosis
==External links==
* Gordon L. Snider, Nosology for Our Day Its Application to Chronic Obstructive Pulmonary Disease, American Journal of Respiratory and Critical Care Medicine Vol 167. pp. 678-683, (2003). fulltext