The Plague of Athens (, Loimos tôn Athênôn) was an epidemic that devastated the city-state of Athens in ancient Greece during the second year (430 BC) of the Peloponnesian War when an Athenian victory still seemed within reach. The plague killed an estimated 75,000 to 100,000 people, around one quarter of the population, and is believed to have entered Athens through Piraeus, the city's port and sole source of food and supplies. Much of the eastern Mediterranean also saw an outbreak of the disease, albeit with less impact.
The war, along with the plague, had serious effects on Athens' society. This resulted in a lack of adherence to laws and religious belief; in response, laws became stricter, resulting in the punishment of non-citizens claiming to be Athenian. Among the victims of the plague was Pericles, the leader of Athens. The plague returned twice more, in 429 BC and in the winter of 427/426 BC. Some 30 pathogens have been suggested as having caused the plague.
Sparta and its allies, with the exception of Corinth, were almost exclusively land-based powers, able to summon large land armies that were very nearly unbeatable. In the face of a combined campaign on land from Sparta and its allies beginning in 431 BC, the Athenians, under the direction of Pericles, pursued a policy of retreat within the city walls of Athens, relying on Athenian maritime supremacy for supply while the superior Athenian navy harassed Spartan troop movements. Unfortunately, the strategy also resulted in massive migration from the Attic countryside into an already highly populated city, generating overpopulation and resource shortage. Due to the close quarters and poor hygiene exhibited at that time, Athens became a breeding ground for disease, and many citizens died. In the history of epidemics in war time, the 'Plague' of Athens is remarkable for the limitation of the affliction to one side as well as for its influence on the ultimate outcome of the war. The Athenians thought that the Spartans could have poisoned their water supply to kill them in order to win the Peloponnesian War.
