demons
Many people in the ancient and medieval worlds believed that evil spirits could cause illness.
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)
ἔνθα καὶ ἐμοὶ ἐπιδημεῖν τηνικαῦτα ξυνέβη. ἐγίνετο δὲ ὧδε. φάσματα δαιμόνων πολλοῖς ἐς πᾶσαν ἀνθρώπου ἰδέαν ὤφθη, ὅσοι τε αὐτοῖς παραπίπτοιεν, παίεσθαι ᾤοντο πρὸς τοῦ ἐντυχόντος ἀνδρὸς ὅπη παρατύχοι τοῦ σώματος, ἅμα τε τὸ φάσμα τοῦτο ἑώρων καὶ τῇ νόσῳ αὐτίκα ἡλίσκοντο.
and I happened to be living there at the time. This is how it came. Many people saw apparitions of demons in every kind of human shape. Those who met them believed that the man they had run into struck them, on whatever part of the body it happened to be. The moment they saw this apparition, they caught the disease.
ὕστερον δὲ οὐδὲ τοῖς φίλοις καλοῦσιν ἐπακούειν ἠξίουν, ἀλλὰ καθείρξαντες αὑτοὺς ἐν τοῖς δωματίοις, ὅτι δὴ οὐκ ἐπαΐοιεν προσεποιοῦντο, καίπερ ἀρασσομένων αὐτοῖς τῶν θυρῶν, δειμαίνοντες δηλονότι μὴ δαιμόνων τις ὁ καλῶν εἴη.
Later, they would not even answer when their friends called. They locked themselves in their rooms and pretended not to hear, even while their doors were being pounded, plainly afraid that whoever was calling might be one of the demons.