According to tradition, there were four things seen by the young Prince Siddhartha which moved him so deeply as to give up his comfortable worldly life. He saw an old man, a sick man, a corpse, and a yogi. The first three of these brought to life for him the notion of impermanence, and the fourth inspired him to the search for the Absolute. Although too young and healthy to directly experience sickness and decrepitude, he nonetheless deeply perceived their role in human existence.
Unfortunately, most of us are not so perceptive, and it is necessary to have some personal experience before we can stop, and evaluate our life, and our direction. I have talked to several people who have had a heart attack, and in many cases heard that it was only after that experience that they started to appreciate each moment of their lives. Several of these people were actually grateful for the heart attack, for it made clear to them that their value system was perverted.
For Zen students being sick is only another opportunity to pursue clarity, and as such it is no different from any other kind of Zen practice. In fact "sickness practicing" is extremely valuable, because even for practicing people it often takes the threat of the loss of their body, or its ability to function well, before they can significantly slow down their desire mind. In the Blue Cliff Record, we have an interesting case about "sick practicing":
Great Master Ma was unwell. The temple Housemaster asked him, "Master, how has your venerable health been lately?"
The Great Master said, "Sun Face Buddha, Moon Face Buddha."
To attain the meaning of "Sun Face Buddha; Moon Face Buddha" is to attain "sick practicing." To attain "sick practicing" is to attain our true self, that which has no sickness or health, no life or death. Then sickness is not our enemy; any sickness from the common cold to AIDS or angina. Any experience provides us with another opportunity to grow in wisdom, love and compassion.