When his daughter Margaret was diagnosed with schizophrenia, Fisher had her treated at the New Jersey State Hospital at Trenton, whose director was the psychiatrist Henry Cotton. Cotton believed in a "focal sepsis" theory, according to which mental illness resulted from infectious material in the roots of teeth, bowel recesses, and other places in the body. Cotton also claimed that surgical removal of the infected tissue could alleviate the patient's mental disorder. At Trenton, Margaret Fisher had sections of her bowel and colon removed, which eventually resulted in her death. Irving Fisher nonetheless remained convinced of the validity of Cotton's treatment.