Shitsudou (失道) is a sickness, which the kirin suffers when a ruler of a kingdom loses the way and fails to govern properly. It manifests through dark facial marking and progressive physical weakness, which gets worse with time and causes the kirin to die. If the kirin dies, the ruler will lose the mandate of heaven and eventually die within a year of the kirin's death. In the case of the king of Kou, he only lives two months after Kourin's death.
Shitsudou is almost always fatal, because when a ruler loses the way, it is most unlikely that the ruler will reform. Depending on the severity of the shitsudou, a kirin may be able to recover and choose another king. There is one other case where shitsudou did not ultimately cause the death of the kirin: the most recent Hourin and her king were both beheaded during the rebellion in Hou. The series accounts for two cases where the kirin survives shitsudou: the current Keiki and the current Sairin. In both cases, their rulers (Jokaku of Kei and Shishou of Sai respectively) abdicated their thrones after realizing that their kirin were dying and killed themselves. In the cases of Keiki and Sairin, they were able to recover and find another ruler for their kingdoms.