When you see a sick person, the Buddha says that you should regard that person, like the aged person, as a messenger from the devas.
And this messenger, too, is delivering two messages.
The first is that you, too, are subject to illness.
The proper response is to learn not to look down on those who are already sick, for they’re simply showing you what the human body—your human body—is capable of. For every part of the body, there are many possible diseases.
At the same time, you have to learn to be heedful in your actions so that, at the very least, you have a refuge of good kamma to hold you in good stead when you yourself fall ill. At best, you want to reach the attainment that’s not affected by illness at all, so that you’ll be able to live in peace even when sick.
The second message is that all beings are subject to illness.
This means that when you fall sick, you’re not being unfairly singled out for any particular injustice. It’s a natural process happening to everyone; so, it would be childish and immature to react to your illness with feelings of resentment or irritation.
Here again, you have to be heedful so that your emotions don’t blind you to the opportunities that still lie open to you, even when ill, to do good in your thoughts, words, and deeds.
As with aging, the Buddha regards illness as nothing out of the ordinary. It’s everywhere and always.
This reflection by Ajaan Geoff is from the book, Undaunted: The Buddha’s Teachings on Aging, Illness, Death, & the Deathless, Chapter Five, “Illness.”