The original Natural Hygienic Doctors stated this from the middle 1800's; especially Dr John H Tilden who wrote a book on the subject called "Toxemia Explained..The Basic Cause of All Disease".
Without proper living habits the body becomes enervated, leading to enervation and the accumulation of metabolic waste material within the body. Disease is just the effort by the body to rid itself of this waste material.
In their view the best way to achieve this is by physiological rest or "Fasting" followed by correct living habits.
From personal experience I know this to be true: in 1975 I had numerous health problems which were totally eradicated by a 26 day water-fast supervised by a Dr Keki Sidhwa followed by correct living habits in accord with the laws of nature. Upton Sinclair (the famous novelist) was quoted as saying after a lengthy Water Fast that he had discovered a state of super-health unknown to humans.