Infantile physique and qi are still not fully developed and the viscera are delicate, therefore infants are susceptible to the invasion of pathogenic factors. The exogenous pathogenic factors invade weifen (defensive system) either through the skin or through the mouth and nose, affecting the dispersing and descending functions of the lung and giving rise to the stagnation of the lung qi. The exogenous pathogenic factor is Pathogenic wind generally, or it is intermingled with pathogenic cold or heat. This causes wind and cold, or wind and heat to obstruct the lung.

When the wind and cold are transmitted into the interior and transformed into heat, or the exogenous pathogenic wind heat transmits into the interior, it will give rise to excessive pathogenic heat and sputum transformed from the scorching body fluid. In this case, the sputum intermingled with heat will obstruct the lung, bringing about the symptoms of fever, cough, asthma with short breath, flapping of ala nasi, accumulation of sputum, etc.. If the pathogenic factors are excessive and the healthy qi is deficient, other zang organs may be affected. The lung controls respiration and convergence of vessels. If lung qi is obstructed, the lung will fail to perform its function of governing vessels, affecting the normal circulation of qi and blood. Furthermore, in this case, the cardiac function of controlling blood and vessels wilt be affected, resulting in deficiency of the heart qi, or even deficiency of the heart yang, giving rise to blood stasis.

If the excessive heat invades the pericardium internally to stir up the liver wind, symptoms of pathogenic factors invading Jueyin meridian will appear. At the late stage of pneumonia, weak body resistance after elimination of pathogenic factors, or weak body resistance with lingering pathogenic factors will turn out to be the two transformed results of lung heat due to yin deficiency or qi deficiency of the lung and spleen based on the differences in infantile individual constitution.