When a TCM physician consults patients, the first step is to differentiate exogenous disease (illnesses originating form outside of the body) from endogenous disease s (illnesses originating form inside the body); because, they have different principles of diagnosis. There are two common methods for identifying exogenous diseases. These are "syndrome differentiation according to the six meridians" and "syndrome differentiation according to defense(wei), vital energy(qi), nutrient(ying) & blood(xue)" discussed in the next section.

The six meridian method was developed early in the Eastern Han Dynasty by Zhang Zhongjing (150-219AD) in the book of Shanghanlun (Discourse on Fevers and Miscellaneous Illnesses), which is a monograph about many types of external infectious diseases. Later this book became an important guide for identifying this kind of diseases. The method investigates the pathological changes over the course of an exogenous disease, especially for cold induced diseases referred to as "shanghan".

Based on the distribution of the three yang meridians (Tai Yang, Yang Ming, Shao Yang) and the three yin meridians (Tai Yin, Shao Yin, Jue Yin), symptoms and signs of exogenous diseases are classified into six stages that correspond to one of these particular meridians. The three yang meridians are located on the exterior parts of the body and as a disease or evil progresses the disease's symptoms will progress along the more interior yin meridians. Therefore, a TCM practitioner is able to identify what stage of a disease a patient has by the signs and symptoms displayed upon one of these six meridians. The more interior the disease has progressed the more severe the illness and damage to the organs.

In cold-induced exogenous diseases (shanghan), the external cold and wind evils enter the body through the skin and muscular striae. They pass along the meridians and eventually attack the organs. There are particular pathways of transmission for the evils that invade the body. They usually pass from the exterior parts of the body to the interior and in this process gradually deplete the body of its qi (vital energy).