contagion
The idea that disease passed from person to person by touch, breath, or objects, long before anyone knew about germs.
The Plague at Athens, and Pericles Answers the Athenians
Thucydides, History of the Peloponnesian War, Book 2 (sections 47–65) · after 430 BCE (events of 430–429 BCE)
δεινότατον δὲ παντὸς ἦν τοῦ κακοῦ ἥ τε ἀθυμία ὁπότε τις αἴσθοιτο κάμνων (πρὸς γὰρ τὸ ἀνέλπιστον εὐθὺς τραπόμενοι τῇ γνώμῃ πολλῷ μᾶλλον προΐεντο σφᾶς αὐτοὺς καὶ οὐκ ἀντεῖχον), καὶ ὅτι ἕτερος ἀφ’ ἑτέρου θεραπείας ἀναπιμπλάμενοι ὥσπερ τὰ πρόβατα ἔθνῃσκον· καὶ τὸν πλεῖστον φθόρον τοῦτο ἐνεποίει.
The most terrible part of the whole disaster was the despair. As soon as people felt themselves falling sick, they lost all hope, gave themselves up far more easily, and stopped fighting. Just as terrible, they caught the disease from one another as they nursed each other, and they died like sheep. This caused more deaths than anything else.
The Plague at Constantinople
Procopius, History of the Wars, Book 2 (chapters 22–23) · c. 550 (events of 542; Books 1–7 made public in 551 at the earliest)
διὸ δὴ ἅπαντες αὐτοὺς οὐχ ἧσσον ἢ τοὺς πονουμένους ᾠκτίζοντο, οὐχ ὅτι τῷ λοιμῷ ἐπιέζοντο ἐκ τοῦ προσιέναι (οὔτε γὰρ ἰατρῷ οὔτε ἰδιώτῃ μεταλαχεῖν τοῦ κακοῦ τοῦδε τῶν νοσούντων ἢ τῶν τετελευτηκότων ἁπτομένῳ ξυνέβη, ἐπεὶ πολλοὶ μὲν ἀεὶ καὶ τοὺς οὐδὲν σφίσι προσήκοντας ἢ θάπτοντες ἢ θεραπεύοντες ταύτῃ δὴ τῇ ὑπουργίᾳ παρὰ δόξαν ἀντεῖχον, πολλοὶ δὲ τῆς νόσου ἀπροφασίστως αὐτοῖς ἐπιπεσούσης εὐθὺς ἔθνησκον), ἀλλ’ ὅτι ταλαιπωρίᾳ πολλῇ εἴχοντο.
So everyone pitied the nurses as much as the sick. It was not that the nurses caught the plague by coming near the sick. (No doctor or anyone else caught this disease by touching the sick or the dead. Many people who kept burying or nursing even strangers held out in that work against all expectation. Meanwhile, many others fell sick for no visible reason and died at once.) People pitied the nurses because of the great hardship they endured.