The infantile skin is not tight. Therefore, the seasonal pathogenic factors and the six exogenous pathogenic factors may all transform into fire, which invades ying and blood systems, impairing the collaterals and giving rise to blood extravasation. If yang collaterals are impaired, blood will extravasate exteriorly, causing skin ecchymoses; if yin collaterals are affected, blood will oveflow interiorly, resulting in hematemesis and epistaxis. Besides, congenital insufficiency or weak physique after recovery causes deficiency of qi and blood.
On this occasion, blood will extravasate from vessels, developing into purpura. Finally, dysfunctions of the liver in storing blood, the spleen in keeping blood circulating intravascularly and the heart in propelling blood may result in failure of the liver to store blood, failure of the spleen to keep blood circulating intravascularly and blood stasis due to qi stagnation, bringing about skin eruption, hematemesis and epistaxis, abdominal pain, etc.