When a new disease comes to light, AIDS, SARS, and most recently COVID-19, the health experts and the media bandy about words like epidemic and pandemic. Today, COVID-19 has been described as on the verge of becoming a global pandemic.
The word pandemic with relation to disease means affecting all the people. pan meaning all, demos meaning people or district, Greek pandemos. So medically, speaking we see it as either potentially affecting everyone or more usually affecting every possible region of the world, in the sense of a global pandemic.
An epidemic has a similar meaning, the epi means among, and the demos might refer to people or a district with people, so among the people of a given district. It is usually used to refer to an outbreak in a specific region or among a group of people, hence the word epidemiology, the study of epidemics, outbreaks of disease in a given area or among a group of people. There is also usually some implication of the rapid spread of a given disease in an epidemic.
In contrast, a disease that is endemic is usually confined to a specific geographical group of people or region. The en simply meaning in.