"Epidemic" and "pandemic" are both used to describe widespread outbreaks of a disease but there are some subtle differences between the two words.
An epidemic (from Greek epi- upon + demos people) is a classification of a disease that appears as new cases in a given human population, during a given period, at a rate that substantially exceeds what is "expected," based on recent experience (the number of new cases in the population during a specified period of time is called the "incidence rate").
A pandemic (from Greek πᾶν (pan, “all”) + + δῆμος (dēmos, “the people”) is an epidemic that spreads across a large region (for example a continent), or even worldwide.
edit Differences between an epidemic and a pandemic
Simply put, when an epidemic gets out of hand, it is called a pandemic. This has 2 nuances:
- Geographical spread
- An epidemic that is not localized to a city or a small region but spans a larger geographical area can be called a pandemic.
- Incidence rate
- An epidemic may be localized to a small region but the number of people affected may be very, very large compared to what is "expected". In this case, it can be called a pandemic even if its geographical spread is not very large. For example, let us say that a disease has an "expected" rate of infection of 15%. When 40% of the population of a state is infected, we have an epidemic on our hands. When 75% of the population is infected, it has reached pandemic proportions.
edit Video explaining the differences
In this video, Dr. Kenneth Alexander, chief of pediatric infectious diseases at the University of Chicago Medical Center, explains the differences between an outbreak, an epidemic and a pandemic and others questions about the swine flu outbreak.