Origins
The word 'pandemic' collapses geography into demography. It tells you not where a disease has spread but who has been engulfed by it: all the people, everywhere. Its two Greek components — 'pan' (all) and 'dēmos' (people) — form a word that is the outer limit of a hierarchy of disease-scale terms, all built on the same Greek political vocabulary.
The Greek 'dēmos' (δῆμος) was a rich and contested word in ancient Athens. It meant the people — but specifically the free citizens of a political community, conceived as a group with shared territory and shared governance. The 'demos' was the political subject of democracy (demos + kratos, people-rule), and also the local district or ward. The PIE root behind 'dēmos' is *deh₂- (to divide, to share out), and the 'demos' was the people conceived as those who share — who share land, laws, and civic identity.
From 'dēmos,' Greek formed a cluster of words about where a disease is in relation to a people. 'Endēmos' (ἔνδημος) meant 'within the people, native to a place' — hence 'endemic,' a disease that persists within a local population as a constant presence. 'Epidēmos' (ἐπίδημος) meant 'upon the people, visiting a place' — hence 'epidemic,' a disease that falls upon a population from outside, spreading quickly but not necessarily everywhere. 'Pandēmos' (πάνδημος) meant 'of all the people, belonging to the whole populace' — hence 'pandemic,' a disease that has spread to all peoples, across all geographical boundaries.
Latin Roots
The word 'pandemic' in its modern epidemiological sense emerged in the seventeenth century in medical Latin and French, though 'pandēmos' in Greek had broader uses — Aphrodite Pandemos was Aphrodite worshipped by all the people, the common goddess, as opposed to her celestial aspect. The word was applied to disease with increasing precision through the eighteenth and nineteenth centuries as global travel made it possible for diseases to spread across continents.
The Greek prefix 'pan-' (all) is extraordinarily productive in English: 'panorama' (all-view), 'panacea' (all-cure), 'Pan-American,' 'pandemonium' (all-demons, coined by Milton for Satan's capital), 'pantheism' (all-god), 'pantheon' (all-gods), 'pantomime' (all-imitation), 'panoply' (all-armor). The prefix expresses totality and universality — qualities that, in the context of infectious disease, are catastrophic.
The COVID-19 pandemic of 2020 brought the word into daily usage for the first time since the 1918 influenza pandemic, which killed an estimated fifty million people. The World Health Organization's declaration of a pandemic on March 11, 2020 was a formal taxonomic act: the disease had moved from epidemic to pandemic, from upon some people to upon all people. The Greek grammar of disease had become, once again, the most important sentence in the world.