Han-era medical physicians believed that the human body was subject to the same forces of nature that governed the greater universe, namely the cosmological cycles of yin and yang and the five phases. Each organ of the body was associated with a particular phase. Illness was viewed as a sign that qi or "vital energy" channels leading to a certain organ had been disrupted. Thus, Han-era physicians prescribed medicine that was believed to counteract this imbalance. For example, since the wood phase was believed to promote the fire phase, medicinal ingredients associated with the wood phase could be used to heal an organ associated with the fire phase. To this end, the physician Zhang Zhongjing (c. 150–c. 219 AD) prescribed regulated diets rich in certain foods that were thought to cure specific illnesses. These are now known to be nutrition disorders caused by the lack of certain vitamins consumed in one's diet. Besides dieting, Han physicians also prescribed moxibustion, acupuncture, and calisthenics as methods of maintaining one's health. When surgery was performed by the physician Hua Tuo (d. 208 AD), he used anesthesia to numb his patients' pain and prescribed a rubbing ointment that allegedly sped the process of healing surgical wounds.