History of the Peloponnesian War

The same summer Hagnon son of Nicias, and Cleopompus son of Clinias, who were colleagues with Pericles, took the army which he had employed, and went straightway on an expedition against the Chalcidians Thrace-ward, and Potidaea, which was still being besieged: and on their arrival they brought up their engines against Potidaea, and endeavoured to take it by every means.

But they neither succeeded in capturing the city, nor in their other measures, to any extent worthy of their preparations: for the plague attacked them, and this indeed utterly overpowered them there, wasting their force to such a degree, that even the soldiers of the Athenians who were there before were infected with it by the troops which came with Hagnon, though previously they had been in good health. Phormio, however, and his sixteen hundred, were no longer in the neighbourhood of the Chalcidians [and so escaped its ravages].

Hagnon therefore returned with his ships to Athens, having lost by the plague fifteen hundred out of four thousand heavy-armed, in about forty days. The soldiers who were there before still remained in the country, and continued the siege of Potidaea.