How does Marcus Aurelius explain suffering as prescribed by nature, like a doctor's remedy?

Marcus draws the comparison directly. He writes that when we hear that Aesculapius, the god of medicine, prescribed a man horse riding, cold baths, or going barefoot, we understand this as something fitted to his health. We should understand it the same way when we hear that the nature of the universe prescribed a man disease, mutilation, or loss (Meditations 5.8). The word prescribed carries the same meaning in both cases: what happens to a person is fitted to him according to his fate, the way a doctor fits a treatment to a patient.

The comparison rests on his idea of fitness or harmony. Just as squared stones fit together into a wall or a pyramid, everything that happens fits into the single coherent structure of the universe. Marcus calls this fitness one thing running through all of it. Necessity, or destiny, is built up out of all existing causes the way the universe itself is built up out of all bodies. So when something is brought to you, it was fitted to you specifically, the way a particular regimen is fitted to a particular patient.

The common misreading is to hear this as a claim that suffering does not really hurt, or that Marcus is telling himself to feel nothing when illness or loss arrives. That is not what he says. He calls the doctor's prescriptions themselves disagreeable, and admits we accept them only in the hope of health. The comparison is not that hardship feels good. It is that we already know how to live with something unpleasant when we trust the source: we drink the bitter medicine because we trust the doctor's aim. Marcus is asking you to extend that same trust to nature as a whole, not because nature flatters you, but because it would not bring anything to any person that was not useful for the whole (Meditations 5.8).

This only works if you also hold the view Marcus states elsewhere: that the gods, if they exist, do not involve you in real evil, and that they have put the means in your power to avoid genuine harm (Meditations 2.11). What nature prescribes can include disease, dishonor, or pain, but these are not the kind of evil that can make you worse, since they fall equally on good and bad people alike (Meditations 2.11). Only your own judgments and choices can do that. So the prescription metaphor and the indifferents doctrine work together: nature hands you circumstances, but whether they harm you depends on what you do with them, not on the circumstances themselves.

What changes if you take this seriously is not your feelings in the moment, but your interpretation afterward. You do not have to pretend a diagnosis or a loss is pleasant. You can call it disagreeable, the way Marcus calls the doctor's cold baths disagreeable, and still receive it as fitted to something larger than your immediate comfort. This does not ask for cheerfulness. It asks you to judge the outcome the way you judge a course of treatment: by whether it serves health, not by whether it is comfortable to swallow.

A concrete way to use this: the next time something unwelcome happens that you did not choose and cannot undo, ask yourself the question a patient asks about a hard treatment, not is this pleasant, but what is this fitted to accomplish. You may not always see the answer. Marcus did not ask for certainty about the details, only trust in the fitness of the whole.