"A pandemic is an epidemic occurring on a scale that crosses international boundaries, usually affecting people on a worldwide scale."

"An epidemic is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time."

Simply existing in many different countries isn't enough; there has to be large-scale spread in those countries.

In addition, the disease must have a noticeable impact on normal human life. It will not count if a new pathogen emerges and spreads virulently across the globe, but is completely harmless and there are no significant efforts to prevent its spread.

If anyone feels this definition is too vague for you to be comfortable betting, tell me how I could effectively reduce the ambiguity and I'll update this description as necessary.