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. Under the direction of Pericles, the Athenians 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 adding many people from the countryside to an already well-populated city, introducing a severe crowding factor as well as resource shortages. Due to the close quarters and poor hygiene exhibited at that time Athens became a breeding ground for disease and many citizens died including Pericles, his wife, and his sons Paralus and Xanthippus. In the history of epidemics the 'Plague' of Athens is remarkable for its one-sided affliction and bias on the ultimate outcome of a war.
In his History of the Peloponnesian War the historian Thucydides, who was present and contracted the disease himself and survived, describes the epidemic. He writes of a disease coming from Ethiopia and passing through Egypt and Libya into the Greek world—a plague so severe and deadly that no one could recall anywhere its like, and physicians ignorant of its nature not only were helpless but themselves died the fastest, having had the most contact with the sick. In overcrowded Athens, the disease killed an estimated 25% of the population. The sight of the burning funeral pyres of Athens caused the Spartans to withdraw their troops being unwilling to risk contact with the diseased enemy. Many of Athens' infantry and expert seamen died, as well as their general Pericles. After the death of Pericles, Athens was led by a succession of leaders Thucydides described as incompetent or weak. According to Thucydides, not until 415 BC had Athens recovered sufficiently to mount a major offensive, the disastrous Sicilian Expedition.