Galen, Constitution of the art of medicine 297k
“Certainly, whenever there is some mass or malignancy of humors or a blockage or some wasting force invades the body, there is a danger previously absent that a person will get sick and there are times when this risk is severe. These types of causes are hard to diagnose because the person doesn’t feel any pain yet.
This is like the infection from a rabid dog: there’s no particular sign in the body before the person afflicted comes near madness. These kinds of causes make it necessary, therefore, that the doctor inquire from patients about everything that happened to them.”
ὅταν γὰρ ἤτοι πλῆθός τι χυμῶν ἢ φαυλότης ἢ ἔμφραξις ἢ φθαρτικὴ δύναμις ἐγγίνηται τῷ σώματι, κίνδυνός ἐστιν ὅσον οὔπω νοσῆσαι τὸν ἄνθρωπον, ἔσθ’ ὅτε δὲ καὶ κινδυνεῦσαι τὰ ἔσχατα. δυσδιάγνωστα δ’ ἐστὶ τὰ τοιαῦτα τῶν αἰτίων ὡς ἂν μηδέπω λυποῦντα τὸν ἄνθρωπον. ὥσπερ ἀμέλει καὶ ὁ τοῦ λυττῶντος κυνὸς ἰός, οὗ σημεῖον ἴδιον οὐδέν ἐστιν ἐν τῷ σώματι περιεχόμενον πρὶν ἐγγὺς ἥκειν λύττης τὸν δηχθέντα· καὶ δὴ καὶ πυνθάνεσθαι τῶν τοιούτων αἰτιῶν ἀναγκαῖόν ἐστιν τὸν ἰατρὸν αὐτῶν τῶν πασχόντων ὑπὲρ τῶν | συμπεσόντων αὐτοῖς ἀπάντων.