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.
Epidemiology is the study of the incidence and patterns of disease with a view to finding means of prevention or control. Traditionally, epidemiological methods were applied to communicable diseases – those, such as cholera or measles, that can be passed from one person to another. However, modern epidemiology is equally concerned with the role of environmental and lifestyle factors in disease causation: for example, the link between lung cancer or between asbestos and the cancer known as mesothelioma.