<unk> s may be classified by cause, pathogenesis (mechanism by which the<unk>  is caused), or by symptom(s). Alternatively,<unk> s may be classified according to the organ system involved, though this is often complicated since many<unk> s affect more than one organ. A chief difficulty in nosology is that<unk> s 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<unk>  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<unk> s is the World Health Organization's ICD. This is periodically updated. Currently, the last publication is the ICD-11.
Disease
Precancerous condition
Cancer
Psychopathology
Drug
Drug 
Cancer cluster
Suboptimal health