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
1 2
Students: Are you brave enough to let our tutors analyse your pronunciation?
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.
Site Hint: Check out our list of pronunciation videos.
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:
Students: We have free audio pronunciation exercises.
Thanks Man you helped me out in my science class you are my hero! i hope you have a very nice day. . 

Your friend, Danny Frost
Show more