 His enthusiasm for the scientific medicine that was taking hold at the opening of the 20th century led him to an unshakable belief that mental illness of all kinds was the result of untreated infections in the body.Based on the observation that patients with high fever often turn delusional or begin hallucinating, Meyer introduced the possibility of mental illness (then viewed as the cutting edge concept of scientific medicine) being a biological cause of behavioral abnormalities, in contrast to eugenic theories which emphasized heredity and to Freud's theories of childhood traumas.Cotton would become the leading practitioner of the new approach in the United States.