Spring-wind, summer-heat, summer-fire, later summer-dampness, autumn-dryness, and winter-cold are the six variations in the climate of the four seasons. They are also known as the "six climatic factors" or the "six exogenous qi."

The human body has the ability to adapt to climatic variations. However, when bodily resistance is too low to adapt to climatic changes or if there is an abnormal altering of the weather which surpasses the body's adaptability, then the six climatic qi will become pathogenic factors bringing about the occurrence of disease. Therefore the climatic qi are all considered to be exogenous pathogenic factors.

Diseases which are not caused by exogenous pathogenic factors, but have symptoms similar to the syndromes of wind, cold, summer-heat, dampness, and dryness are termed internal heat, internal damp, internal dryness, and internal fire. These endogenous pathogenic factors are the outcome of dysfunctions of the zang and fu organs.