Here is a good article about the origin of the term ‘epidemic’
As I said in response to your other question about ‘when’, I’m certain that observant people recognized clusters of similar illness within small communities well before they had anything like a germ theory of illnesses. We can’t possibly know what the first instance was.
We can though use modern science to look at diseases that were spreading long ago. For instance they have found evidence of tuberculosis in mummies:
Here is some work on plague closer to us in both time and geography: