ταῦτα δηλώσω αὐτός τε νοσήσας καὶ αὐτὸς ἰδὼν ἄλλους πάσχοντας –> I shall describe those symptoms, since I myself had the disease and witnessed as well what others were suffering

[billberg23]

Thucydides 2.48.3.5 on the viral epidemic in Athens, 430 BCE:
ἐγὼ δὲ οἷόν τε ἐγίγνετο λέξω, καὶ ἀφ' ὧν ἄν τις σκοπῶν, εἴ ποτε καὶ αὖθις ἐπιπέσοι, μάλιστ' ἂν ἔχοι τι προειδὼς μὴ ἀγνοεῖν, ταῦτα δηλώσω αὐτός τε νοσήσας καὶ αὐτὸς ἰδὼν ἄλλους πάσχοντας → I'll report what sort of illness it turned out to be, so that anyone who notices the symptoms and recognizes them in advance won't fail to identify the disease should it ever reappear. I'm going to describe the symptoms because I myself had the disease and witnessed as well what others were suffering.

[spiros]

Very pertinent!