The ancient world's experience of epidemic disease — before modern medicine, before germ theory, before effective quarantine or treatment — shaped the demography, economics, and religion of every major civilization in ways that have been clarified by recent epidemiological history.

The Antonine Plague (165–180 AD), probably smallpox or measles, is estimated to have killed 5–10 million people across the Roman Empire — perhaps 10% of the total population. The simultaneous Plague of Cyprian (249–262 AD), possibly viral hemorrhagic fever, killed at an even higher rate. The demographic and economic impact of these pandemics on the 3rd-century crisis has been substantially reassessed in recent scholarship.

The impact of European epidemic diseases on pre-Columbian American populations after 1492 is the most extreme example of epidemic disease as a historical force. Indigenous American populations had no prior exposure to smallpox, measles, typhus, influenza, and other Eurasian diseases, creating conditions for catastrophic mortality rates — possibly 50–90% in some regions within a century of contact. The population collapse preceded and facilitated European conquest; understanding the conquest without the epidemic is historically impossible.

Egyptian medical papyri, Hippocratic case histories, and Thucydides' clinical description of the Athenian plague (which is too detailed to allow confident modern diagnosis) all represent attempts to understand epidemic disease empirically. The inability to identify causes (germ theory was millennia away) did not prevent the development of practical public health measures — isolation of the sick, cleaning of water supplies, removal of refuse — that had real if partial effectiveness.