History of the Peloponnesian War

The same summer Agnon the son of Nicias and Cleopompus the son of Clinias, who were joint commanders with Pericles with that army which he had employed before, went presently and made war upon the Chalcideans of Thrace and against Potidaea which was yet besieged.

Arriving, they presently applied engines and tried all means possible to take it, but neither the taking of the city nor anything else succeeded worthy so great preparation. For the sickness coming amongst them afflicted them mightily indeed and even devoured the army. And the Athenian soldiers which were there before and in health catched the sickness from those that came with Agnon. As for Phormio and his sixteen hundred, they were not now amongst the Chalcideans.

And Agnon therefore came back with his fleet, having of four thousand men in less than forty days lost one thousand and fifty of the plague. But the soldiers that were there before stayed upon the place and continued the siege of Potidaea.