From AN 5.57, translated by Bhikkhu Bodhi:
Bhikkhus, there are these five themes that should often be reflected upon by a woman or a man, by a householder or one gone forth. What five? [second of the five]
(2) A woman or a man, a householder or one gone forth, should often reflect thus: ‘I am subject to illness; I am not exempt from illness.’
and later
(2) And for the sake of what benefit should a woman or a man, a householder or one gone forth, often reflect thus: ‘I am subject to illness; I am not exempt from illness’? In a state of health beings are intoxicated with their health, and when they are intoxicated with their health they engage in misconduct by body, speech, and mind. But when one often reflects upon this theme, the intoxication with health is either completely abandoned or diminished. It is for the sake of this benefit that a woman or a man, a householder or one gone forth, should often reflect thus: ‘I am subject to illness; I am not exempt from illness.’
and later
(2) This noble disciple reflects thus: ‘I am not the only one who is subject to illness, not exempt from illness. All beings that come and go, that pass away and undergo rebirth, are subject to illness; none are exempt from illness.’ As he often reflects on this theme, the path is generated. He pursues this path, develops it, and cultivates it. As he does so, the fetters are entirely abandoned and the underlying tendencies are uprooted.
Moving on from the closely related subject of ageing, we come to a reflection on illness. When we are well, we could often be called “intoxicated” with health. We take the attitude that we are superior to those who are not healthy; we may pity them, if we think about them at all. It is hard, when we’re well, to remember having been ill. Likewise, when we’re ill, we fear the illness will never end; it becomes hard to recall how it felt to be healthy and strong.
For many of us, there is more fear associated with illness than there is with ageing or even death. Death seems remote until we’re right next to it; but sickness is an ever-present threat. This may be why we are so vulnerable to sellers of false remedies for sickness.
When a young person comes down with a serious illness, we are reminded that there is no justice in the world, at least on a scale that humans can make sense of. We don’t like acknowledging that sickness is just as much a part of our life story as birth and death are, and that it’s outside of our control. We cling to the delusion that we can keep ourselves safe from the dangers of nature, when the truth is that we can only control our own actions and reactions.
It came as a shock to me that everything, even trees (even our dear, 200-year-old elm tree), has a natural, finite lifespan. No part of nature escapes the “inconveniences” of ageing, becoming sick, and dying. We’re all in this together.