Yogācārabhūmi Śāstra: Volume 34 (22)

How to observe the impermanent nature of the changes caused by diseases in the internal body. That is, by observing oneself or others previously without disease, peaceful and strong. Later, observing oneself or others suffering from severe illness, experiencing intense pain, with various bodily sufferings, as previously explained in detail. Again at another time, seeing them without disease, peaceful and strong. Having seen this, one thinks thus: Such conditioned phenomena are impermanent by nature.

How to observe the impermanent nature of the changes arising from diseases in the body? By observing that oneself or others previously had no disease, with peace and strength in body and mind. Later seeing oneself or others suffering from severe illness, being very ill, experiencing great pain. Then again observing that oneself or others are no longer ill, with peace and strength in body and mind. After observing these phenomena, one has such a thought in mind: Such conditioned phenomena are truly impermanent by nature.