The Han dynasty's Guaqi （hexagram qi） theory， inheriting the tradition of divination and studies of

Yijing

， is rooted in the concepts of heaven-human resonance and correspondence， as well as the yin-yang and five-element theories of disaster prediction. It constructs a divinatory， symbol- and numerology-

based study of

Yijing

that integrates and governs the qi dynamics of heaven and earth. The concept of disaster prediction， expressed through the timeliness or deviation of the 24 solar terms within the four seasons and eight solar terms to forecast wind， rain， cold， and heat using symbol and numerology-based divination， was introduced into medicine， as seen in

Huangdi Neijing

（

Yellow Emperor's Inner Canon

） and

Shanghan Lun

（

Treatise on Cold Damage

）. The disease concepts associated with the eight trigrams and the twenty-four solar terms include hexagram imagery， solar terms， phenology， and the human meridian system， fully reflecting the early understanding of epidemic qi theory centered on abnormalities in weather phenomena and seasonal patterns. The Han dynasty's Guaqi （hexagram qi） theory embodies a disease model of the "unity of heaven and human"， which integrates temporal， atmospheric， and climatic factors. It also gave rise to early medical concepts of epidemic diseases， including Shanghan （cold damage） and Wenbing （warm diseases）. Furthermore， the five-element theory of disasters and epidemics laid the theoretical foundation for the development of the theory of Wuyun Liuqi （five movements and six qi）.