what are the differences between the two?
are they interchangable?

epidemic a widespread occurence of a disease, or occasionally of some other undesirable thing eg an epidemic of bad grammar

pandemic The prefix 'pan' refers to 'all' (from Greek). A pandemic is a big epidemic, typically involving the entire world or perhaps a whole country. Unlike 'epidemic', 'pandemic' is only used to refer to diseases.

Best wishes, Clive
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the prefix 'epi' has some meaning of 'top', 'big' as in 'epicenter', where an earth quake starts and where the force of the quake is at its peak. what is the meaning of other part: 'demic'? population? people? or disease?

'demic'? population? people? or disease? It's from the Greek demos = people. As in democracy, demographic, etc.

In the medical community, yes epidemic indicates a widespread episode of disease, while pandemics are global or spanning several countries.
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Thank you Clive
Your exampe, Clive, of an epidemic of bad grammer would in all actuality be more of a pandemic.
Here is an answer from another list:
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