One of the most common bases for classifying disease is according to cause.
Degenerative diseases occur as a result of the natural aging of the body tissues.
Finally, a wide range of diseases are attributed to, or at least influenced by, emotional disturbances. Most diseases occur as a result of a combination of both internal and external conditions, i.e., an interaction between the body and the environment. In ancient times disease was ascribed to supernatural, spiritual, and humoral factors. The discovery by Louis Pasteur and others of the role played by microorganisms in infection and the study of cellular pathology by Rudolf Virchow in the 19th cent. were of the utmost importance in establishing the true nature of disease. Licensed from Columbia University Press.