An epidemic is the occurrence of a disease in a geographically localized population over a limited period of time; it usually refers to infectious disease which spreads from case to case by carriers. Epidemics arise from importation of infection, after environmental changes favoring infectious organism or due to altered host susceptibility.
A pandemic is an epidemic of very large or worldwide proportions. Infectious disease is said to be endemic in an area if cases are continually occurring there. Travel through endemic areas may lead to epidemics in non-endemic areas.