Enchiridion, Section 1

"Things not in our power include the body"

Perfectly nice, healthy people get sick all the time. We can try to influence our health, through things like eating a low-carb diet and exercising, but it's a mistake to think that will determinstically lead to a healthy life. Bad things happen to good people.

Writing circa 125 AD, Epictetus knew this because nobody knew anything about germs or cancer. It was a truism then, but it's easy to forget with modern medicine, with X-rays and blood testing, but we still don't know everything and we still can't screen for everything. Even in 2019 people get incurable, chronic diseases like Multiple Sclerosis and Crohn's.

Your body isn't in your control, so influence it where you can, but accept what happens.

"Remember then that if you imagine ... what is naturally another's is your own, you will be hampered, you will mourn, you will be put to confusion, you will blame gods and men; but if you think that only your own belongs to you, and that what is another's is indeed another's, no one will ever put compulsion or hindrance on you, you will accuse none, you will do nothing against your will, no one will harm you, you will have no enemy, for no harm can touch you."

Mistaking what belongs to someone else as your own will always make you unhappy. You will try to exert influence on whatever it is, but because it belongs to someone else, you'll be disappointed in the result. But if you keep your wits about you and only try to control those things that belong to you, nobody will have power over you.

Always ask yourself when you're feeling frustrated about something whether it actually belongs to you, or if you're trying to take responsibility for something that isn't yours.