Chills or rigor is shaking that results from an increase in fever. It occurs when cytokines and prostaglandins are produced by the immune system which increase the temperature set point in the hypothalamus. The body reacts as if it had hypothermia and starts shivering in order to generate more body heat to increase body temperature. The patient perceives that they are cold until the new set point is reached. Partly for this reason, physicians should not rely on the patient's subjective perception of their own body temperature and should instead measure it directly.