of diseases, "incident to a whole people or region," 1660s, from Late Latin pandemus, from Greek pandemos "pertaining to all people; public, common," from pan- "all" (see pan-) + dēmos "people" (see demotic). Modeled on epidemic; OED reports that it is "Distinguished from epidemic, which may connote limitation to a smaller area." The noun, "a pandemic disease," is recorded by 1853, from the adjective. Related: Pandemia.

*dā-, Proto-Indo-European root meaning "to divide."

It forms all or part of: betide; daimon; Damocles; deal (v.); deal (n.1) "part, portion;" demagogue; demiurge; democracy; demography; demon; demotic; dole; endemic; epidemic; eudaemonic; geodesic; geodesy; ordeal; pandemic; pandemonium; tidal; tide (n.) "rise and fall of the sea;" tidings; tidy; time; zeitgeist.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dati "cuts, divides;" Greek dēmos "people, land," perhaps literally "division of society," daiesthai "to divide;" Old Irish dam "troop, company;" Old English tid "point or portion of time," German Zeit "time."