The main causes of the onset of chickenpex are epidemic wind-warm pathogen, dampness, heat and toxin. In winter and spring, epidemic wind-warm pathogen, dampness, heat and toxin attack the defensive system, make the lung fail to disperse and descend. In this case, dampness and heat combined with each other to penetrate the skin, giving rise to chickenpox. Healthy qi fails to conquer pathogenic factors. This is the internal cause of the onset of chickenpex. The infantile lung is delicate.

The lung is associated with the defensive system because it dominates the skin and hair and has its orifice in the nose. Therefore, the lung is the first target to be attacked by epidemic wind-warm pathogen, dampness, heat and toxin through the mouth and nose. Persistent insufficiency of the infantile lung, together with the weak external defence fails to drive the pathogenic factors out of the body. This makes infants liable to be attacked by epidemic wind-warm pathogen, dampness, heat and toxin.