A method for treating a disease by returning the body to a natural condition. The method adjusts a parameter of a thermotherapy for treating or preventing the disease, disability, or condition of a patient and comprises (A) a step of measuring the redox potential (or pH value) of the patient and (B) a step of determining the parameter of thermotherapy suited to the patient according to the redox potential. Considering returning the biopotential to a range called homeostatic potential, a therapeutic potential of -75 mV to -90 mV (the potential in the cell is -75 mV) the absolute value of which is higher than that of the cell is given. It has been found that this therapy effects a cure.