The occurrence of exogenous disease is usually caused by invasion of pathogenic factors which first attack the surface of the body, then invade the collaterals with scanty defensive qi, and gradually get into the internal part of the body. The occurrence of endogenous disease is caused either by insufficiency or imbalance of qi, blood, yin and yang. If qi, blood, yin and yang are insufficient, the meridians and collaterals will become empty; if qi, blood, yin and yang have lost balance, the meridian qi will be stagnated or in disorder, leading to the occurrence of disease.

Disease is usually transmitted along the meridians. The exogenous pathogenic factors are transmitted from the external to the internal, from the collaterals to the meridians, and from the meridians to the viscera. Thus the pathological changes of the viscera can be transmitted by the meridians. For example, disease of the zang-organs can be transmitted to the fu-organs and the disease of fu-organs can be transmitted to the zang-organs by the meridians in external and internal relationship with each other. 

On the other hand, the diseases of the five zang-organs can be transmitted among them because of the multiple relationships among the meridians and collaterals. For example, the liver disease can be transmitted to the lung and the stomach; the kidney disease can be transmitted to the heart and the lung; etc. Besides, disease of the internal organs can be transmitted to the surface of the body, leading to pathological changes of the related constituents, organs and orifices. 

For example, angina pectoris may lead to tenderness on medial side of the upper limb along which the heart meridian runs; the disease caused by stomach-fire may cause swelling pain of the gums through which the stomach meridian runs; up-flaming of liver-fire may cause ocular disease, etc