section_path: Classification
section_name: Classification
paragraph_text: Classification
  
  Diseases may be classified by cause, pathogenesis (mechanism by which the disease is caused), or by symptoms. 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).
  
  Classical classification of human disease derives from the observational correlation between pathological analysis and clinical syndromes. Today it is preferred to classify them by their cause if it is known.
  
  The most known and used classification of diseases is the World Health Organization 's ICD. This is periodically updated. Currently, the last publication is the ICD-11.