Thucydides, Peloponnesian War 3.87
“As winter started coming on, the disease afflicted the Athenians a second time—even though it hadn’t totally disappeared before, there was still a period of relief. Then it lingered no less than a year when the first encounter was two. The overall result was that nothing overwhelmed the Athenians and sapped their power more than this.
No fewer than 4400 hoplites died from the ranks along with three hundred cavalry. And the number of the rest of the masses that died was never discovered.”
Τοῦ δ’ ἐπιγιγνομένου χειμῶνος ἡ νόσος τὸ δεύτερον ἐπέπεσε τοῖς ᾿Αθηναίοις, ἐκλιποῦσα μὲν οὐδένα χρόνον τὸ παντάπασιν, ἐγένετο δέ τις ὅμως διοκωχή. παρέμεινε δὲ τὸ μὲν ὕστερον οὐκ ἔλασσον ἐνιαυτοῦ, τὸ δὲ πρότερον καὶ δύο ἔτη, ὥστε ᾿Αθηναίους γε μὴ εἶναι ὅτι μᾶλλον τούτου ἐπίεσε καὶ ἐκάκωσε τὴν δύναμιν· τετρακοσίων γὰρ ὁπλιτῶν καὶ τετρακισχιλίων οὐκ ἐλάσσους ἀπέθανον ἐκ τῶν τάξεων καὶ τριακοσίων ἱππέων, τοῦ δὲ ἄλλου ὄχλου ἀνεξεύρετος ἀριθμός.
The famous plague is described at 2.47-55 and first fell on the Athenians in 430 BCE. Three years later, after a respite, it returned.