In Chinese medicine, health is represented as a balance of yin and yang. These two forces represent the bipolar manifestation of all things in nature, and because of this, one must be present to allow the other to exist. Hence, where there is above there is below, whatever has a front also has a back, night is followed by day, etc.. On an emotional level, one would not know joy had they never experienced pain.
It is important to note that the balance of yin and yang is not always exact, even when the body is healthy. Under normal circumstances the balance is in a state of constant change, based on both the external and internal environment.
For example, during times of anger, a person's mood is more fiery, or yang, and yet once the anger has subsided, and a quiet peaceful state is achieved, yin may dominate.
This shift in the balance of yin and yang is very natural. It is when the balance is consistently altered, and one (be it yin or yang) regularly dominates the other, that health is compromised, resulting in illness and disease.
Traditional Chinese medicine practitioners attempt to determine the exact nature of the imbalance, and then correct it through the use of acupuncture, herbal remedies, exercise, diet and lifestyle. As the balance is restored in the body, so is the health of the individual.