Epidemiology Definitions

An epidemic is a rate of disease that reaches unexpectedly high levels, affecting a large number of people in a relatively short time. Epidemic is a relative concept: a small absolutely number of cases of a disease is considered an epidemic if the disease incidence is usually very low. In contrast, a disease (such as malaria) is considered endemic if it is continuously present in a population but at low or moderate levels, while a pandemic describes epidemics of world-wide proportions, such as influenxa in 1918 or HIV/AIDS today.