During the Spring and Autumn period, the state of Chu attacked Xiao, a city of Song. The people of Xiao captured two Chu ministers, including Prince Bing, to force a retreat. The King of Chu immediately agreed and ordered his army to withdraw. But the Xiao people killed the two ministers anyway. Enraged, the King of Chu surrounded Xiao tightly. He personally toured the troops, encouraging soldiers and checking on the wounded. Inspired, the army fought with renewed vigor, and Xiao's fall was only a matter of time.
During the Xiao campaign, a commander named Huan Wushe, who had ties with the Chu official Shen Shuzhan, secretly begged his friend for help. Shen Shuzhan asked, "Do you have any wheat yeast?" Huan replied, "No." Shen pressed, "Do you have any medicinal herbs?" Huan again answered, "No." Shen then said, "Without these remedies, what will you do if you fall ill like a river fish?" Shen was hinting that Huan should escape by water, but unable to speak openly in the army, he used this coded question. Huan, failing to grasp the meaning, simply kept saying no.
The next day, Xiao was captured. Following Shen Shuzhan's advice, Huan Wushe hid in a dry well and escaped the disaster.
Fish living in water are prone to internal ailments. The idiom "He Yu Fu Ji" (River Fish Belly Ailment) also refers to diarrhea.
Source: *Zuo Zhuan*, "Duke Xuan, Part Two"
Meaning of the Idiom: Later, the Chinese idiom "河鱼腹疾" came to describe how a river fish's belly ailment refers to diarrhea.