"A plague so great as this, and so dreadful a calamity, in human memory could not be paralleled." This passage comes courtesy of the Greek historian Thucydides in one of the well-known passages from his History of the Peloponnesian War. Granted, this was around 430 B.C. and the world had yet to witness, well, basically all of recorded history. But the Plague of Athens was catastrophic nonetheless, especially to Greek forces who were in the midst of a war with Sparta. Modern researchers have conjectured about the nature of the plague, with some saying it was typhoid, typhus fever, smallpox or even anthrax. But its true nature may never be known. Virtually all of the information we have comes from Thucydides, who traced its roots to Ethiopia and said a third of the city's people perished as a result. He's as good a source as any, considering Thucydides himself also contracted it.