"After I turned 28, I had convinced myself that there exists an art of hygiene and I followed its precepts the whole rest of my life. The result was that I never got sick except for the occasional ephemeral fever; but, surely it is possible for someone to preserve themselves completely if they have secured a life of freedom.* This already became clear in the previous sections, and it will become even clearer in the sections that follow should you choose to pay attention. For I say no buboes** can be produced in anyone who has rigorously prepared for health, simply if their body is free from of each kind of excess residue, both in quantity and quality."
ἀλλὰ μετά γε τὸ εἰκοστὸν ὄγδοον ἔτος ἀπὸ γενετῆς ἐμαυτὸν πείσας, ὡς ἔστι τις ὑγιεινὴ τέχνη, τοῖς προστάγμασιν αὐτῆς ἠκολούθησα παρ' ὅλον τὸν ἑξῆς βίον, ὡς μηκέτι νοσῆσαι νόσημα μηδέν, ὅτι μὴ σπάνιόν που πυρετὸν ἐφήμερον. ἔστι δὲ δήπου καὶ τοῦτον αὐτὸν φυλάξασθαι τελέως, ἐλεύθερον ἑλόμενον βίον, ὡς ἔν τε τοῖς ἔμπροσθεν ἤδη γέγονε δῆλον ἔτι τε μᾶλλον ἔσται σαφὲς ἐν τοῖς ἐφεξῆς εἰρησομένοις, εἰ θέλοι τις προσέχειν τὸν νοῦν. ἐγὼ γάρ φημι μηδὲ βουβῶνα δύνασθαι γενέσθαι τοῖς ἀκριβῶς παρεσκευασμένοις εἰς ὑγείαν, εἴ γ' ἀπέριττον αὐτοῖς ἐστι τὸ σῶμα τοῦ γένους τῶν περιττωμάτων ἑκατέρου, τοῦ τε κατὰ τὸ ποσὸν καὶ τοῦ κατὰ τὸ ποιόν.
Galen, De sanitate tuenda, 5.1, 6.309-10 Kühn
*Galen is referring to something he said a bit earlier: that he gets an occasional fever because sometimes he works too much. In other words, he is not free, since he is dedicated to his practice. If you are free, all the more reason you should remain healthy if you follow his advice.
**not quite sure why Galen is singling out buboes here.