《黄帝内经·素问·疟论》
热极之时，则疟邪又回入于阴经，使阳气又虚，阳气虚便出现寒冷。
衰则气复反入，入则阳虚，阳虚则寒矣。
When the fever reaches extreme, the pathogenic factors of malaria re-enter the yin channel, causing the yang-qi to become insufficient, and when yang-qi is insufficient, chill will occur.