(especially of medicine) of disease or anything resembling a disease. Attacking or affecting many individuals in a community or a population simultaneously.

An epidemic outbreak of influenza.
epidemic

Of or relating to a disease (or anything resembling a disease) constantly present to greater or lesser extent in a particular locality.

Diseases endemic to the tropics.
Endemic malaria.
Food shortages and starvation are endemic in certain parts of the world.
endemic, endemical