Through long term clinical practice, the ancient Chinese realized that there are many factors which may bring about imbalances in the human body and thus disease; climate abnormalities, pestilence, emotional stimulation, injury by irregular diet or overstrain, trauma, insect-bited, etc., plus pathological products of disease outcome, such as blood stasis, phlegm-humor, etc. All of these contribute to imbalances within the human system.

The etiology of traditional Chinese medicine used clinical manifestations as evidence, i.e., through the analysis of symptoms and signs of a disease, one can find its causative factors. This is technically termed "checking syndromes to find causative factors of a disease." For our study of etiology, we must concern ourselves with the properties of pathogenic factors and the characteristics of how and why they cause disease.

Traditional Chinese medicine holds that the occurrence of a disease not only depends on exogenous factors, but more importantly is decided by body resistance. Chinese medicine terms all exogenous pathogenic factors as xie qi, while the body's is relatively weak, xie qi will have an opportunity to attack and Suwen records, "If a pathogenic factor attacks the body, then the zheng qi must be weak." Furthermore, "When zheng qi exists in the interior, the pathogenic factor will be unable to interfere."

Therefore the invasion of xie qi is due to the insufficiency of zheng qi, this is the root cause. Xie qi is necessary condition for the occurrence of a disease. The development, transformation, and prognosis of a disease depend on the forced balance of zheng qi and xie qi.