The healthy qi, one kind of qi that constitutes the body and maintains life activities, exists in all the internal organs and meridians and ensures their normal functions. When pathogenic factors attack the body, healthy qi fights against pathogenic factors and protects the body. That is why it is literarily called "righteous qi" in TCM.

Only when healthy qi is superabundant can it fight against pathogenic factors. The state of healthy qi is decided by the congenital kidney-qi and the acquired gastrosplenic qi. Since essence, blood, body fluid, yin and yang can transform into each other, they directly influence the state of the healthy qi. If healthy qi is insufficient, the body resistance will be weakened, giving rise to the invasion of pathogenic factors.

Pathogenic factors, the factors that directly disturb the normal physiological functions of the body, include six abnormal climatic factors, pestilence, abnormal changes of the seven emotions, improper diet, overwork, over-rest, phlegm, rheum and blood stasis, etc.

      

The occurrence of disease results from invasion of, pathogenic factors into the body and struggle between pathogenic factors and the healthy qi. The invasion of pathogenic factors into the body is exclusively caused by insufficiency of the healthy qi. Since pathogenic factors can only attack the body when healthy qi is deficient, they are called "deficient pathogenic factors". The combination of "deficient pathogenic factors" with the deficiency of healthy qi is prerequisite to the occurrence of disease, which is described in Huangdi Neijing as "when two conditions of deficiency combine with each other, pathogenic factors begin to attack the body".