"Pandemic" is a transparent compound, once you know its parts. From Greek pan ("all") plus dēmos ("people"), the word means literally "affecting all the people." It is the geographic big brother of epidemic, which uses epi- ("upon") and refers to a disease that has fallen upon a specific population.
Demos, the Public
The Greek dēmos originally meant a deme — one of the small administrative districts of Athens — and by extension the common people who lived in them. It is the same root that gives us democracy ("rule by the people"), demographic, demagogue, and endemic. The opposite of endemic — "in among the people" — is epidemic: "upon the people," a sudden outbreak.
Into English
The word entered English in the mid-17th century, used initially by physicians and theologians describing diseases that crossed national borders. The 1918 influenza outbreak, often called the "Spanish flu," firmly established pandemic in everyday use, and the COVID-19 outbreak of 2020 cemented it as one of the most-used English nouns of the early 21st century.
A Suffix That Travelled
The Greek -demic suffix has wandered well beyond medicine. We now have infodemic (an outbreak of misinformation), and the prefix pan- attaches to a long list of nouns: pan-African, panorama ("see all"), panacea ("all-healing"), pandemonium ("all the demons"). The literal meaning of "pandemic" — "concerning everyone" — turns out to be a useful template for naming things that include or affect everybody.