This koan is called “Dongshan is unwell.”

When Dongshan was not feeling well, a concerned monk inquired, “Teacher, you are not feeling well. Is there anyone who doesn’t get sick?”
Dongshan said, “Yes, there is.”
The monk continued: “Does the person who doesn’t get sick take care of you?”
Dongshan said, “I have the opportunity to take care of the one.”
The monk then asked: “What happens when you take care of ‘that one?’”
To which Dongshan replied, “At that time, I don’t see the sickness.”

Please consider these Discovery Questions:

Who is “that one?”

Why does Dongshan no longer experience sickness?

Source: Book of Serenity, Case 94